A defense prefers to play combo coverage when the ball is on the hashmarks because a definitive wide and short side are created and the defense is trying to better cover one of those areas. Generally the defense will use zones in the combo coverage to the wide side unless a trips or diamond (three or four receiver) formation presents itself to the short side. The defense will use the field to their advantage because the sideline to the short side limits what an offense can employ in terms of pass routes in particular out-routes, so the weak side cornerback will play with inside leverage on the receiver. The defense will also change the matchups in man coverage after the snap, this is known as a "bracket coverage". For instance if the offense lines up twins (two receivers) to one side against underneath man coverage and the outside receiver runs an inside route and the inside receiver runs an out route; the defenders can "pass off" the receivers to each other for better positioning on the route and to throw off the quarterback's read. These type of coverages need to be beat by play calling, audibles and execution primarily. The offense tries to beat this coverage by stretching the coverage and isolating a specific matchup in the field. The offense will also use a lot of motion to make the defense change responsibilities and better detect underneath coverage. A heavy dose of plays from motion will break the defense from using combo coverage but the quarterback has to execute those plays to establish the advantage.
Cover 1 man free coverage:
Type of coverage: Combo, one safety in deep middle zone, man underneath coverage
Strengths: Deep help in the middle of the field, pass rush (can rush five), safety in the box, strong side run support, possible inside help if a linebacker doesn't blitz, he can drop into short middle-zone coverage.
Weaknesses: Out routes, inside help, play action, deep outside, linebackers manned up on backs.
The safeties are the main read in this alignment. A safety (usually the strong safety) will be locked in close to the tight end while the free safety will be in the middle of the field just before the snap. The safeties will sometimes rotate, with the free safety and strong safety switching roles, or the free safety dropping back from a closer position to his deep middle just before the snap to add deception. The cornerbacks will have an outside shade to funnel the receivers to the deep middle (to safety help) on long routes. If a receiver motions and a defender follows it should indicate a possible man-free coverage because it is the only man-scheme with a safety lining up in the middle.
How to attack this coverage?:
The basic slant-route is effective against any man coverage because there is no designated help to the inside and the receiver will have position on the cornerback if he gets a good release. In the man-under scheme, the tight end and running backs can be used to clear out the linebackers to allow for a clear passing lane to the slants. One good rule of thumb - man coverage is always about positioning, and the slant route is one of the best ways to gain leverage on a cornerback for a low risk pass pattern. The post corner route is also effective because it can sink the safety into the post, then the receiver can break it back to the sideline away from both the safety and cornerback.
The offense can also freeze the safety with the tight end running down the middle to take the safeties deep help away or at least slow them down from helping out to the deep outside zones. A good example was Braylon Edwards first touchdown catch against the Texas Longhorns in the Rose Bowl.
(Above) Texas was running a cover-2 zone but the point still holds true. The safeties have deep help but get sucked in by Tim Massaquoi and Tyler Ecker running fly routes inside the safeties with Braylon Edwards running the wheel (basically a outside slant, then fly route down the sideline) route to the outside to stretch the deep safeties. The free safety for Texas, Phillip Geigger, has deep zone to Braylon's side and is drawn in by tight end Tyler Ecker's deep route to the inside of his zone. With Ecker pulling him to the inside he is late getting to Edwards, who wisely stretched the zone all the way to the sideline and was able to catch the ball before Geigger could get over to him. The goal was to have Geigger be late in support, and through a perfect play-call, excellent route and great throw by Chad Henne, Michigan exploited the deep safety. The same can be done in cover-1. The offense can send a tight end or running back on a fly route to the inside which draws the safety to him and like wise is late to support anything to the deep outside zones that is thrown to the receivers. It essentially creates a one-on-one matchup between the receiver and cornerback.
The offense also can exploit the mismatch with the linebackers covering the running backs one-on-one. Offensive coordinators will also use shallow drag routes to keep the underneath coverage close while the receivers run routes to exploit the space between the linebackers and safety. The play-action pass works too because the free safety is a deep middle cover man and may not be able to get to the deep outside zones; therefore if the offense can get the corner to bite on the play action, the receiver will have a step on both the safety and corner.