Charlie Harbison instructs them to NEVER let them go up the seam unchallenged.
Now we start into a series of pattern recognition examples, to show you how the coverages are supposed to work. In the example diagrams I've created below, I'm assuming the coverage is a basic Cover 3, no strong or weakside rotation in the coverages. The assignments for a particular player in the coverage can change but I've picked the usual CB-FS-CB for the 3 deep zones all separated into 1/3s. Cover 3 is better against any odd receiver sets (2x1, 3x1, 3x2) and the balanced alignment gives good interior run support. It gives your defense the benefits of Cover 1 , run strength inside, without the hazard of individual matchups in man-coverage.
I've made the assignment of a standard 4-3 defense, and that the SS and WLB take the flats, while the MLB and SLB take the intermediate hook/curl zones, but this really does not make a difference. It is the player's own technique and assignment that matters. It also doesn't matter if its Nickel or Dime coverage, the position taken by a LB/SS would be the same as the extra defensive back. As such, his technique only depends on the assignment given for the coverage called on the sideline.
Flat - Stays inside the alignment of #1 receiver to that side but stays outside everyone else, so any receiver that comes toward the sideline must not get wider than the flat defender is playing. As we defined before, it means he can't let anyone cross his face without breaking on the route. His aiming point in his drop is 10 yards deep in the seam and anyone that comes nearby should be rerouted by force. The man playing the flat or curl/flat area is generally the guy with the toughest assignment because of the amount of yards he must cover.
Hook/Curl - Original drop inside the #2/3 receiver, and must never let someone go up the seam undefended. Based on the release of the #2 receiver, he stays on the inside edge of the numbers. If he's on a TE, he must get his hand on him. Exactly what he does to him depends on whether the TE starts outside or inside from the LOS. If he goes outside, the defender drops 10 yards and immediately looks for a Smash pattern.
Deep Coverage - Corners take anyone man/man who goes deeper than 12 yards from the LOS, while the middle coverage defender (FS/SS) watches the seams to either side.
Our article on pass concepts shows the several concepts we've defined so far, that are used in the diagrams here, and the article on terminology and communication is necessary to understand some of the terms I'm using.
From Saban's Defensive Playbook, the basic Cover 3 Matchup Zone:
By the diagram you can see Saban calls them Weak/Strong Hook zones, and Curl/Flat for the outer zones. Its just terminology. In his Cover 3, the Strongside Corner plays the curl-flat if he calls for a jam on #1 (bump), and in that case the SS takes the deep 1/3.
Remember when we count receivers, we count from the outside-in 1-2-3 on each side. This is done pre-snap, and defenders align off of that. But, once the play starts a defender actually is assigned 1-2-3 post-snap. By that I mean if the X or Z receiver (who will be #1 pre-snap) breaks inside while the slot guy breaks outside, then the slot guy becomes the final #1 and X/Z becomes the final #2.
BrophyFootball goes into pattern recognition by deep coverage by reading the number of drop steps by the QB and the direction his shoulders are aimed in this video (not his eyes). Pattern reading is the technique by which defenders are taught to 'guess' the breaks of their assigned receivers based on what they see other receivers doing. If someone goes deep, expect another short, or vice versa. In this video you'll see what the FS or deep coverage defenders are taught to read, then they can break quicker on the ball.
Flat defender takes his initial 45-degree drop here, staying under the #1 receiver. Remember he's trying to make the QB throw over him and defending the #1 with his body, while keeping an eye on the FB breaking out. As soon as the FB crosses his face, he breaks on the flats.
CB drops into his zone intially, watching #1 and #2 to his side up the seam. If theres no threat deep, he jumps the curl route.
The H/C defender (MLB below) takes his 45-degree drop, and having seen no threat, keeps going back to the curl route. As you can see the #1 receiver has 3 guys to deal with in this pattern.
H/C defender sees the #2 going up the seam and stays inside both him and the #3, and takes the TE man/man. Had the TE broken outside, the H/C defender would be on the RB (i.e., he takes the final #3, after the play has developed).
Flat defender begins his drop, staying inside the #1 (Z) to his side, then when the RB crosses his face or also threatens the seam, he must take him man/man. Based on the inside break of the TE, he should expect the RB to break outside and stay in the flat area as a release valve. He must always keep outside leverage (meaning he comes from outside/sideline-in).
Strongside corner maintains outside leverage and takes his drop initially, but then pressures the post route and sticks him man/man. The middle safety watches both seams first, and after those are clear he works to the post route.
Strongside: I'll define it as the Y-Z-RB side.
Flat defender (SS) drives to the #1 receiver who runs a hitch in the flat. The H/C defender starts dropping to cover the #2 up the seam, but once he's gone (and he is watching #1, so he expects #2 to go deeper) he jumps on #3 underneath.
The corner to this side watches #1 and #2, and once he sees #1 stop his route, he immediately should suspect #2 to come deep and that he must pick him up.
Flat defender works to the flat zone and picks up #1, while the H/C defender (MLB) stays inside the #2 during his drop. Once the weakside Corner sees no threat deep or the seam, he works to the ball.
The free safety drops and reads the QB, protecting the seams to either side.
Flat defender drops to the flat zone and picks up the RB when he crosses his face. The H/C defender starts his drop up the seam but then takes the first receiver that breaks inside, and tries to wall him off. The deep corner takes the deepest threat, which in this case is the TE on a flag route.
Flat defender starts his drop underneath the #1 receiver who is running a Dig route, and keeps inside leverage on him. Once he sees someone cross his face he jumps him in the flat (#2).
The H/C defender (MLB) runs with the #1 receiver on the Dig, remember he's supposed to cover any inside breaker into his zone. If the Z couldn't be walled off and breaks underneath, he must keep him in front of him, and try to stay under that Dig route.
The Corner closes on the most dangerous threat he sees, while the FS is reading the QB and breaks on any throw.
This is a Flow pattern, so they are overloading the strong side.
Flat defender takes the RB that goes furthest outside into the flats, the final #1.
The SLB (H/C) knows that the coverage has shifted behind him to his side, because he recognizes Flow, so there are now 2 defenders immediately behind him. The coverage is not 1/3-1/3-1/3 any longer, its more 1/4-1/4-1/2. The Corner and Safety now have 1/4ths and the backside CB has 1/2 the field. SAM's assignment is the final #2, so the SS takes the final #1 outside (the A above) and SAM takes the 2nd (the RB above, who blocks initially).
The corner to this side drops and waits for any deep threat or looks for a crosser. If the receiver in the flat breaks deep, its his responsibility.
H/C defender knows that since the coverage shifted, he has only one deep helper. He'll cover the next inside breaking receiver (the TE) man/man. The flat defender sees no threat immediately, and takes his drop until he recognizes the final crosser underneath.
Weakside corner covers the post route man/man. The safety must always respect the post route, and will pick that up.
This is also a Flow pattern.
Flat defender stacks and slides with the (initial #1) Z receiver until he sees the RB come out and crosses his face, then he takes the Back (final #1).
Since this is flow, again the H/C defender on the strongside knows he has 2 deep helpers, so he lets the first 2 receivers go by (A, Y) and works towards the final #2 (the Z).
The Corner takes his 1/3 drop, then closes over the middle on the deeper crosser (Y) who is the intended receiver on this play in most cases.
On the weakside, the H/C defender only lets the first go by (A), gets depth, then picks up his man (the TE, final #3).
The WLB covering the flats takes his initial drop to force the QB to throw to X over his head, but recognizing flow alignment initially, expects a crosser. He must take the first one to come his way and works from depth.
Both the safety and weak corner have to respect the post route.
Notice the difference here is the RB route.
The only flat threat is the Z, so the SS takes him. The H/C covers either #2 or #3, using the leading/trailing rules so there must be communication between the two middle underneath defenders here. The way I've drawn it, he walls off #2 and then drops to #3, but he could just drop to #3 up the seam. If the TE released outside that would usually be the right play.
The corner works on the Post route by the TE.
H/C linebacker takes whomever his teammate didnt take, whomever is open of the inside breakers. Neither of them can be wrong and must communicate.
Flat defender drops initially then sees the back, and takes him man/man, while the weak Corner takes his receiver man/man.
The FS drops and looks for any threat to the middle of the field.
A Verticals pattern.
Flat defender takes an outside drop and stays on the outside hip of #2 (A) up the seam (I've drawn it incorrectly). The LB takes the inside hip of #3 and gets his depth up the seam. The Corner must get a drop between #1 and #2 and then react to the ball or the deepest threat if he passes him.
The FS is alert to the Trips formation (3 rec to strongside) and expects the throw to the strong side.
Flat defender (WLB) takes the outside hip of the back up the seam, while the CB takes #1 man/man all the way.
The reason why the CB takes the inside hip is that he can use the boundary as help: just push em out of bounds.
Another flow pattern, but this time the popular post-dig combination.
Flat defender sticks to the 2nd man through his zone, the "A" in this case. He doesnt pay any attention to the RB when he comes out. The H/C Sam LB is stacked first, then watches for the checkdown safety valve pass to the RB.
The corner sticks the outside hip of the receiver running the post, meaning he is the outside help and the FS is the inside help. Only when your CB is very good man/man do you change the assignment of the FS to take the Dig route instead.
H/C LB passes off the 1st crosser (Y) to the flat defender, recognizing he is free to cover him because of the initial flow of the backs. The LB then drives hard to get underneath that Dig route. Notice here that initially either side can be the Dig or the Post, both break around 15 yards here, so he must get depth and see the pattern before he commits and drives hard. This is the tricky part to this coverage.
The weak corner stays behind the Dig route, and in some cases could replace the FS as inside help on the post (meaning the FS drops down to the Dig route).
Now for clips of Alabama and LSU (Saban) running Cover 3.
[h/t to brophy for the videos]