As a preview for both Miami and UNC specifically, and an explanation of what Auburn did, we've decided to go back to the 4-3 defense and explain some basics of it differently than we have before. To do that, we have to explain the concept of the 4-3 when it was thrust onto the forefront of college football in the 1980's by Jimmy Johnson and the Miami Hurricanes.
Up until the 80s, most teams were option teams of some form, and fairly few had serious pro-style passing games. Alabama ran the Wishbone for most of the Bear Bryant era and in the 1980s it was the Switzer Oklahoma teams that ran all over people with the 'Bone. Most college teams ran a 5-2 Monster (PDF download) defense which had begun to morph into the 3-4 now that the pass was becoming more prevalent and since it was more popular in the pro ranks, but Miami really pushed the 4-3 as its currently known. They developed the system now called the "Over" or "Slide" as a way to handle the option teams they faced, but also be flexible to defend against the passing system they ran. I don't know who "invented" the 4-3, and it was around before Johnson became the Defensive Coordinator at Arkansas (Tom Landry ran a 4-3 Flex at Dallas in the 50-80s, and he took it from his playing days with the Giants), but once other coaches saw how Oklahoma State (Johnson's 1st HC job) and Miami handled both, and with Johnson later going to Dallas and winning Super Bowls with the same defense, the 4-3 spread again like wildfire.
After that, Johnson's students and assistants have gone all throughout the country using the same basic system. Butch Davis runs a version at UNC. Randy Shannon runs it at Miami now. Tommy Tuberville was an assistant under Johnson in Miami and has run it at Auburn (Chizik still does) and Texas Tech. Dave Wannstedt runs a less-aggressive version at Pitt. Butch trained Greg Schiano who runs a similar system at Rutgers, and thats just the main players from Johnson's coaching tree. Charlie McBride used the system at Nebraska from the late 80s through the Osbourne years, and Mickey Andrews ditched the old 46 Bear they once ran for a similar system in the 80s. Steele worked for both of the latter two.
I call this defensive scheme the "Over" as opposed to the "Under" that Clemson primarily runs. The main difference schematically between the two is the number of weak spots, called "bubbles", in the defensive front. There are other 4-3 schemes, but nowadays one of these two is almost always the base defense.
What did Johnson change at Miami? Some of this you already know. To stop the wishbone option on the edge, you need fast OLBs and Ends. Johnson decided to recruit Safety-types, 6'-6'2 190-210lb, which are fairly easy to find, and put them at OLB. If you bulk them up without losing their speed, you have more speed on the field, but it doesnt matter how tall they are, they just have to be fast and tackle well. He took the guys who played LB in HS and moved them to DE, again you put more speed on the field. The guys who played DE in HS got moved to DT. Warren Sapp was actually a fullback in HS. That's pretty much how things worked out for him in south Florida, and most teams do the same things today.
To play the Over, you need to recruit only one "true" linebacker, the Mike. Everyone else was converted from the position they played in HS. How many good LBs can you think of that played for the guys mentioned above? They were usually MLBs. You play your best LB at the Mike, who is protected by the NG, and he can make plays all day unless the NG is awful. The NG and 3-technique must still be able to handle a double-team to keep the LB free.
In the 4-3, in general (meaning either Under or Over), you put the Ends down off the shoulder of the OT and angle them to the RB so they can make the Block-Down-Step-Down rule work easier. To reiterate from the article we wrote on the Zone Read, the BDSD rule says that when the OL across from you blocks down (towards the center), you must follow him and step down (towards the center) the LOS. Once you think about how the option works, with linemen constantly veering inside to option off the End, you see how this worked against the wishbone teams of the time. This stops the Dive of the option, which is the basis of the system(s) and makes it go. If the Dive went inside or the QB tried to keep, the End would be right in his face.The faster OLBs could just run around the OL who went to the 2nd level. If they cannot execute the BDSD rule, then the 4-3 system is not a great one to run.
Turns out that doing this with the End lets you recruit a taller guy with more range, more of an athlete than the big heavy Ends of the time, who ended up at DT. When you angle him to the RB in the backfield, you are also putting the End in great position to get the QB sack. So you now have the best of both worlds, a system where you can get pressure with 4 linemen and also handle the wishbone that Oklahoma State and Miami faced often. This is still why teams base out of a 4-3, whether they run an Over or Under as their base. It can handle many things schematically.
Ok now what IS the Over?
Take the defensive linemen and line them up in the 5-3-1-5 technique alignments, except with the Over, you are putting the 3-technique to the strong side. This shifts the DL to the strong side. In the Under, they are shifted weakside and the SAM plays on the Line if there is a TE. As opposed to the 3-4 or 4-3 systems at that time, the linemen have just one gap to worry about, so their goal is just to get upfield and not read the blocks of the OL. They are not there as much to keep the LBs unblocked as they are to make the play themselves. If you think about it, making your DL wait and read a block will make him slower into the backfield, so one-gapping is preferable in terms of aggressiveness as well as being simpler.
To go along with this aggressive philosophy, linemen are stunting very often.
The LBs are aligned over the unmanned gaps, or "bubbles" in this defense, behind the linemen. In its original form, the system is designated right/left, not strong/weak. That means that OLB plays on the same side of the defense always, and doesn't care which side is strong or weak. The right OLB plays on the right side no matter what, same for the left. Likewise, the right DE always plays on the right side, left on left. This lets them get used to seeing things from the same vantage point and reading the same types of blocks from the same personnel.
The primary differences between the Over and Under from an alignment standpoint is at the End and SAM positions, then the two interior linemen. In the Over, the End on the TE aligns wider, in a 9 technique. In the Under, the End stays in a 5 and the SAM comes up to play the 9.
Remember at the time the TE was more of a blocker, particularly for the option, and would come off to scoop-block the SS, and aligning your End at a 9 prevents him from doing so easily. Now, in the passing game, having the End there prevents him from getting clean release.
The interior linemen play a 1 and a 3 alignment, and the only difference between the two systems for them is that in the Over, they shift strong. In the Under, they shift weak. You can see this below. When opponents shift their strength, you just slide the linemen over, thus the other name for this front, the "Slide".
The Over front - DE aligned in a 9 to the strong side, with LBs covering the "bubbles". The ROLB, who here is named WILL, has the B-gap bubble. MIKE is in a 00 technique head-up on the Center, but covers the strongside A bubble. The LOLB is SAM here, aligned over the C-gap bubble.
The Under front - Notice the line has shifted away, instead of being shifted over to the TE, they are undershifted away from him. Here the WILL is on weakside no matter what, covering the weakside A bubble. MIKE has the strongside B bubble. Also notice there are only 2 bubbles now, so this is a stronger alignment vs. run. SAM is always on the TE. The Ends will also play strong or weak, not right/left. The weakside 5 is called the Bandit.
There are now other ways to align the Over, or specific changes for personnel and how offenses block their primary plays, but these are the basic formations and alignments. Alabama, for example, plays the DEs in a 6 and a 4 when they go Over. With a humongous NG, like Terrance Cody, you can just have him handle both A-gaps and move MIKE over to cover for a weaker player beside him. Thats just a few of the ways its been adjusted over the years.
One other feature of this system in its original form is that the Ends are spill players. When they have help behind them, they spill the ball to the help. This means that instead of squeezing, where you basically need to bull rush the blocker backwards into the gap, you are inserting your body into the gap and forcing the ball to cutback or spill outside. Thats where the LBs should be waiting. A squeeze player has to be bigger and strong, but a smaller quick player can get his shoulder turned into the gap and hold his body there to force the RB outside to the OLB who should be coming around to make the tackle. This is still the case for most Under and Over teams.
Miami has since evolved their fronts to play quite a bit of EVEN front, which is in between the Over and Under. Instead of shifting towards or away from the TE, the EVEN front puts the DTs head up on the two Guards in a 2-technique, while the Ends stay in a 5. Everyone is multiple and no one plays the same front in a 4-3 in modern football. Additionally, neither UNC or Miami seem to stick to the R/L designation of the OLBs described above, they do play them strong/weak. When you decide to play Under as your change-up front, youre almost required to have a bigger LB play SAM.
Behind the Over front, the coverage that Johnson installed was Cover 3 and Cover 1. The Cover 3 often takes the form of Quarters on the Field side and Cover 2 on the Boundary side (1/4-1/4-1/2). UNC and Miami base from these but the Canes have shited to more of a Cover 2 team than they once were.
Which is better, or when to use them?
There are 3 bubbles in the Over, 2 in the Under. Either can handle the pass in general, but Under is schematically better against pro personnel, i.e., 1/2 TE and 2 backs. Against I-formation teams, the Under is better because you now have 2 bubbles to worry about and the SAM is on the line, giving you a 5-2 alignment. Under lends itself into a little more Cover 2 Man but Over can run that fine.
Against the spread, the Over as a base is actually better. The reason is that the SAM in an Under system needs to be a real freak of an athlete, but is usually bigger and taller because he plays on the LOS. Think of him as a slightly-lighter DE who has to play pass coverage. If you have to walk him out on a slot WR, there is a mismatch. The only player in the Under who is really meant to be walked out is the WILL, who was recruited as a safety-type. SAM is usually not meant to be on anything but a TE or back. Some coaches do treat him the same as WILL however.
Clemson walks only the WILL out, or switches to Nickel. Auburn, who runs the Over primarily, actually left the 3 LBs in against us more often when we went 4-5 wide. They were not always in Nickel when we made the subsitutions. The reason is because they have recruited Safety-types as the R/LOLB positions, so they can just split them out.
As an example, see below:
If I am set on playing Under Cover 2, then I have only 4 choices to handle this formation when the RB splits out to the slot, or a slot comes in.
1. Leave the slot uncovered.
2. Walk WILL out, keep the same cover 2. This means that the gap he was covering must now be picked up by MIKE, or I have to call a line stunt on the weakside. Having MIKE a 2-gap player makes him read plays slower, and the stunt might get picked up.
3. Walk out the Bandit, and leave WILL in the box.
4. Check out to Cover 1 and drop a Safety on the slot man/man.
None of those is great. Particularly if the outside receivers are good and your Corners need the safety help, or if they can run the ball very well from 1-back sets. Its the same problem if they do it to the strong side.
Why is the Under better vs 2-backs and the Over for 1-back systems?
When a running team sees 3 bubbles, they drool. Think of the Power play, it will usually hurt the Over badly when they run it to the side of the 3-tech. Theres a huge gap between the 3-tech and a 9-tech. Isolation will usually negate their MLB. You want to know why Clemson ran the Power so much against Auburn, this is one reason why. You have to be able to move the DT, which we didn't so well, but the front is schematically weaker vs. this play. However Its not always the X's and O's, but the Jeff's and Joe's.
Now if the offense likes to run Power wide to the C-gap, you have a man on the OTs shoulder always in Under front. The SAM is right beside him on the TE. There is not much gap left anymore. Now if you have a DaQuan Bowers there on the strongside, that hole will disappear. If they double team him, T+TE, then the SAM has a clear run through to the play. He will sometimes get blocked by the FB but only a good one will be able to consistently kick him outside.
So this is the Miami/UNC 4-3 system, but as you can see it uses the same concepts that Clemson's 4-3 uses. We'll save more nuts & bolts of the 4-3 for other articles.