Cornerback is the most technically demanding and acutely unforgiving position on the defense. A rookie corner must adjust to more complicated offenses, more skilled route runners, and tighter, more punitive restrictions on contact. Any mistake, be it misreading the play, erring on a cut, or failing a jam can instantly lead to big yardage or points. That so many corners bust is not surprising. That teams ever draft more evidences the extreme value of top corners and the outright necessity for competent corners.
Cushion: The space between a defensive player playing coverage and an offensive receiver.
In: Towards the center of the field.
Jam: A defensive technique involving striking or shoving a receiver in the legal first five yards beyond the line of scrimmage.
Off: A defensive technique where a defender establishes a moderate to large cushion presnap.
Out: Towards the sidelines.
In football we think of units. Not individual performances, but the interplay of many performances: A running back maximizing a hole created by his offensive line; a tight end exploiting the seam exposed by the split end drawing the safety. Cornerbacks belong to the secondary unit, and though it foolish to think their play and performance is theirs alone, they are among football’s truest mavericks. In tight man coverage, a corner is often solely responsible for maintaining coverage for the duration of the play. Any single failure can end in a touchdown.
Before the snap
To an extent, a corner's presnap proximity to the opposing wide receiver is built into the play. In a Cover 2 Zone, the corner may play three yards off so that their initial zone radius extends further away from the line of scrimmage, rather than wasted in the opponent’s backfield. A corner may also play off facing an unfavorable matchup in man coverage: Kelly Herndon V. Torry Holt. But that off coverage can become a weakness should the play be or be audiblized into a slant, drag or screen. Now, instead of maximizing his zone, the corner has forfeited a step in and precluded any chance of jamming the receiver.
A quality corner must know both the play call and variations within that play call. That same corner working within a Cover 2 might (through careful film study and deduction) read "slant". The corner can’t change the play call, but he can shorten his cushion, adjust inside or outside a receiver or attempt a jam. A corner’s presnap plan is very influential to the outcome of a play. Shortening your cushion can give you the spot you need to jump a slant underneath or cede the space needed to not be beat on a go route.
At the snap
When a play begins, the corner can drop into man or zone coverage immediately or jam the receiver and then drop into man or zone coverage. I’ll cover man and zone coverage in greater depth tomorrow, but today I’ll talk about the types and desired outcomes of jamming a receiver.
The first and most essential goal of jamming is to slow the opposing receiver. Make his route take longer to develop. That time can be essential in developing zone coverage, allowing safeties and linebackers to read and react to the play call and establishing pass rush.
The second and almost equally important goal of a jam is to derail a receiver’s route. If a receiver wishes to slant in, an optimal jam would push him out towards the sidelines. If a receiver wishes to isolate the corner on a go route, the optimal jam might push him in towards the deep safety.
A good jam involves balance, strength and technique. A corner must engage the wide receiver squarely and attempt to jam him by pushing or striking the receiver in the box between his shoulders and hips. Attempt to jam too high and risk exposing your body to being "ripped" away laterally. Jam too low and risk having your hands slapped down before you engage.
A hard jam focuses on maintaining contact as long as possible without consideration for the receivers final position. If a corner wishes to jam in, they must establish themselves outside creating an easier avenue to liberty inside towards the hashmarks. If a corner wishes to jam out, they must do the opposite.
That’s enough for today. We’ll cover after the jam, man coverage, zone coverage, playing the man and playing the ball tomorrow.