The concept of the cover two defense has been around for many years, but it's been only in the past ten years, since the introduction of the cover two by Tony Dungy in Tampa, that it started to gain popularity in both the NFL and college football. The scheme is simple, really, and when played effectively, with key players at key positions, it's extremely difficult to move the ball effectively through the air. The key to a cover two defense is to demand accurate, pinpoint accuracy from the opposing quarterback, which is why it's a perfect scheme against most college quarterbacks.
The easiest way to think about the cover two is like a zone defense. Two safeties and two corners split the field into two halves, playing those halves equally.
Each safety has what is called "deep half responsibility". The safety aligns 12 to 15 yards from the ball, dependent on offensive formation, down and distance, and opposing game plan. He then ‘roams' from middle of the field to the sideline on passing downs and "fills in the alley" on running downs. Covering an entire half of the field seems difficult against the pass, but with solid linebacker play and a good cornerback ‘jam' (more on those later), the safeties' job can be relatively easy. Without either one, the safety is going to have no chance to be successful. As with most other coverage schemes, the safeties responsibility is to not give up the deep ball – keeping everything in front.
The corners responsibility is "jam, release and stay in the flat". A corner in this scheme doesn't have to be a jet with 4.3 wheels (it helps, of course), but he does have to be a guy with good hips and the ability to be physical at the line of scrimmage. Most corners will play from four to five yards off the outside receiver and will adjust if motion comes to his side. Once the ball is snapped, the corner must disrupt the receiver coming off the ball, jamming him to the inside, or "funneling" him to the inside. He CANNOT allow the receiver to take a direct route on his pattern or the safety will have too much room to cover, too soon. If the corner can "redirect" the receiver inside, the area with which the safety has to cover is now cut down significantly. If the receiver takes an outside release the corner "squeezes" him to the sideline as much as possible, giving the safety time to get "off his hash" to play "on top of him". The corner will then play underneath the receiver until another threat comes into his area, the flat. Once that happens, he will slow down, squat and read the quarterback's eyes, reacting to a ball thrown in the flat. Good cover two corners can bait QBs by staying underneath the outside receiver as long as possible, knowing that a flat threat is approaching then breaking on the ball before it's even thrown by the unsuspecting QB.
The linebackers are also key to the cover two defense. The dilemma, though, is that linebackers must think to stop the run first, then pass drop. In the cover two, that pass drop, to alleviate some of the pressure on the safeties, has to be at least 12 to 15 yards in depth. That's tougher than it seems. So, the quicker the run/pass read by the linebacker, the sooner each one can get to his drop zone, the more difficult it is for a quarterback to throw over their heads. In some cases, strong-side linebackers will run with tight ends/inside receivers down the seam to allow the safeties more freedom to play closer to the sideline.
Offenses will try to beat the cover two in two different areas – the "hole" (behind the corner and in front of the safety) and the aforementioned seam (roughly down the hashmark). Throwing the ball into the hole is as tough as it gets, but a receiver who beats the jam easily and gets outside with little resistance gives his QB a bigger hole to throw into…and a nightmare for safeties. Many teams are now trying to use slot receivers to get down the seam to put that safety in a world of hurt, but if the smaller receiver gets bounced around by linebackers, he may never get to a point where he can hurt the deep half playing safety.