Cover two is a zone coverage, played almost exclusively (with the exception of the old VT robber look) out of two high defenses. The "two" in cover two refers to the fact that this coverage contains two deep safeties, splitting the field in half. In the most traditional variety of cover two there will be two corners taking the flats, and then three linebackers taking the hook/curl zones. I played in a cover two defense in high school, a bit as a nose tackle and then three years as a middle linebacker. My formative years as a football fan were spent rooting for the Lovie Smith coached Bears. Cover two is the coverage I know best, by far.
The corners will align outside of the wide receivers, aiming to press them and force them inside at the line of scrimmage. After pressing the receiver the corner will run with them until it’s either time to pass them off to the safety or another receiver/running play threatens the flats, where they’ll rally and (in a perfect world) make a tackle for a short loss.
Cover two corners are also responsible for "forcing" outside runs. What this means is that the lineman and linebackers are attempting to force runs to the outside, where the defense can rally and make a gang tackle. For this to work the corner must be able to keep running plays inside of him, forcing a cutback into the defensive pursuit. If the run is away from the corner he is supposed to begin running at a 45 degree angle towards the opposite sideline and operate as the last person between the ballcarrier and the end zone. Cover two corners tend to be bigger, stockier and stronger than, say, cover three corners as a result of the physical nature of their job.
Cover two safeties have a pretty basic job, cover the deep half of the field. Each safety will align somewhere from ten to fifteen yards from the line of scrimmage, backpedaling if a pass is shown to somewhere around twenty yards deep and keeping his eyes on the quarterback as best he can. From there his job is to be, in the parlance of my old coach, "deeper than the deepest, and wider than the widest" player to threaten his zone, and then break on any throws. Against play side runs the safety has to stay back until he’s absolutely sure there is no threat of a play action, he then comes downhill and operates as a secondary force player, he will almost never make the play at the LOS but the safety is often the difference between a first down and a touchdown. On away side runs the safety serves a similar role as the away side corner, serving as the second to last tackler on broken plays as well as the first tackler if the runner tries to cut back.
Linebackers, regardless of position, have very similar fundamentals and jobs against the run. Each linebacker should begin each play by taking two steps towards the LOS and reading the guard to their side. Guards can really only do a handful of things, run block, pass set, or pull. Versus any run block but a pull the linebacker will fill his gap quickly, looking to penetrate and stop the run for as little a gain as possible or force the ball outside. Versus a pull the linebacker should yell "pull pull pull" loud enough the coach on the opposite sideline can hear it, both alerting his fellow linebackers that a guard is looking to earhole them as well as letting the team know the run is coming behind the pulling guard. The linebacker the guard is pulling away from will then shuffle parallel to the line of scrimmage and attempt to fill any gaps opening on the play side while still forcing the back outside. Over pursuit can, and will, open up cutback lanes. Open cutback lanes mean highlight reels, and not for the defense.
Versus the pass linebackers jobs become a bit more specialized, with outside linebackers serving basically the same role (though, the weak side linebacker often goes against slot receivers and the strong side against tight ends) and the middle linebacker serving a different job. Outside linebackers play the seam/hook/curl zone. What this means is that they will align inside a receiver and attempt to run with him with inside leverage if he runs up the seam. In addition the linebacker is heavily encouraged to hit the receiver and disrupt his ability to get upfield. If no receiver threatens the seam the linebacker will play the deep hooks, finally rallying to any quick curl routes in his zone. The middle linebacker, in traditional cover two, shuffles back and plays the middle hook zone, looking for crossing receivers. In the most common variety of cover two, tampa two, the middle linebacker will play somewhat deeper than either outside linebacker but shallower than the safeties, serving as almost a cover three free safety. This variety came about because traditional cover two is weak against post routes, particularly from the tight end. Cover two linebackers tend to be faster and undersized, particularly middle linebackers, as defenses that base out of this scheme rely on penetration and gang tackling to swarm running plays. This makes it somewhat easier to get short power runs, but it becomes harder to spring big plays.Cover two is a defense based heavily on "bend but don’t break" philosophically. All of the zone defenders, regardless of position, will be attempting to read the eyes of the quarterback so they can break on throws.
In most varieties of cover two, especially on teams that base with four lineman, defensive lineman have a pretty short list of assignments. Most of it can be boiled down to penetrate, penetrate, penetrate. Cover two does not tend to lend itself to zone blitzes, though they can be run, and as a result the lineman are responsible for pressuring the quarterback. The reliance on spilling versus the run emphasizes penetration as well. The goal of a defensive lineman in this scheme is to get a yard or two upfield versus the run and force the back outside. Versus the pass the nose and three technique should look to collapse the pocket, with the three tech especially looking to get pressure on the quarterback. The two interior lineman will be aiming for somewhere around the quarterbacks front shoulder when he sets up to throw. The defensive ends will be looking to speed rush, aiming for the quarterbacks back shoulder while keeping contain should he try to scramble. Lineman on teams that base out of this coverage tend to be undersized, speedy pass rushers, valuing speed at the cost of size and strength.
A series of stunts from the under front (courtesy: Bloggingtheboys.com)
Cover two lends itself to line stunts, with teams using the quickness of their line to create varied pressure in lieu of blitzing. There are a few varieties of cover two, the most common (after tampa two) is known most commonly as "sky" which will involve the corner and safety trading pass and run responsibilities. The trade off here is that the safety will be unable to press at the line of scrimmage, however the trade off between the corner and safety can confuse a quarterback and cause mistakes. Mistakes create turnovers.
There is also cover two robber, wherein the outside linebackers play the flats, one of the safeties and the middle linebacker play hook/curl, and the free safety plays something akin to the spirit of "tampa two" role, reading the release of the tight end or slot to that side and adjusting off of that. In this variety both corners bail at the line of scrimmage to play deep half zones.
Robber Coverage (courtesy: MGOblog.com)
Teams can also zone blitz from cover two, usually sacrificing the middle hook/tampa two zone to send a fifth rusher. This is rarer than the traditional 3x3 fire zone but still plenty viable. There are also pattern read varieties of cover two, but that is another topic for another post.
Cover Two fire zone (courtesy: Blitzology.com)