Primarily due to the outstanding play of players like Dwight Freeney, Robert Mathis, and Raheem Brock, the Indianapolis Colts are known as one of the most dangerous pass rushing teams in the NFL. The amount of pressure the Colts traditionally generate from the defensive end position demands that opposing offensive coordinators adjust offensive line, running back, and tight end blocking assignments to ensure their quarterbacks have time to find an open receiver.
There is a downside to such aggressive pass rushing. The traditional role of defensive ends is to contain the football. The theory behind this responsibility is if defensive ends can turn the ball inside, the ball-carrier is forced to run in the most highly trafficked and patrolled area of the field (lots of defenders). If the ball gets outside of the defensive ends, generally only cornerbacks, safeties, and linebackers on one side of the field have a chance to stop the ball-carrier before he can find open space and pick up chunks of yards.
The television and radio announcers for Colts games will refer to this phenomenon, mentioning that Dwight Freeney or Robert Mathis "take themselves out of the play." The idea behind this statement is that Freeney and Mathis get so far behind the line of scrimmage that running plays, particularly draws (delayed running plays), are allowed to go around the end of the Colts defensive line without even dealing with the pass rushing tandem. There is no way Freeney or Mathis will make the play, so the running back has two fewer defenders to worry about as he heads downfield.
Over the course of the last two seasons, the Colts have addressed this issue by playing bigger players at defensive end on running downs. These players still attempt to generate pressure on the quarterback as their first priority but take up more space and remain in the outside running lanes. This forces running backs to spend more time running laterally before they can make a cut downfield.
While running to the outside and finding a "seam" is desirable, and usually effective against most defenses, lateral runs play to the Colts defensive strength: speed. The rusher is exposed to the defense for so long that the speedy linebackers, safeties, and cornerbacks are capable of heading upfield, stopping the ball-carrier for no gain or for a loss of yards.
Another way the team handles the space created by Mathis and Freeney when they "pin their ears back" and get into the backfield, is to send them inside of the offensive tackles to "blow up" a running play before the running back can make a move. It is not uncommon to see Freeney or Mathis dart quickly behind the offensive line, hitting a ball-carrier before he has the opportunity to make a cut or get to his running lane.
The Colts defensive scheme also plans for the spaces created by penetrating defensive ends by assigning linebackers to those spaces. Sometimes the team will even send a "run blitz" where linebackers or safeties will preemptively hit the holes on the offensive line, searching for a ball-carrier as they come through, on their way to the quarterback. It can be confusing because sometimes it will seem like the blitz is intended only to put pressure on the quarterback, but the blitzes are sometimes designed specifically to plug running lanes.
Depending on the defensive ends that are on the field, the role and responsibility of the position will change in the Colts defense. Freeney, Mathis, and rookie Jerry Hughes will primarily focus on generating a pass rush, attempting to sack the quarterback. They will focus much less often on worrying about hand-offs and running plays. It is not their strategic purpose, though they do attempt to stop the run when they can.
Eric Foster, Fili Moala, Mitch King, and to a lesser extent John Chick are all players who will attempt to play more balanced. They will still attack the line, attempt to generate pressure on the quarterback, and get sacks. However, they will also focus more on "setting the edge" to force running backs into the heart of the defense.
The Colts focus for the defensive end position has changed over the last two seasons, with Larry Coyer and Jim Caldwell putting more emphasis on size. When the big guys are in and Freeney and Mathis are out, the Colts are preparing to stop a running play. When Freeney, Mathis, Hughes, and Chick are on the field together, in some combination, Coyer and company are planning on getting after the quarterback.
One final concept that affects what defensive ends do in the Colts system is called "stunting." Dwight Freeney and Robert Mathis have become so notorious in the NFL as pass rushers that opposing offensive coordinators will spend days or weeks finding ways to limit their impact. One of the most dangerous things about both players is that they are so fast getting to the outside and behind offensive tackles that they can "turn the corner" and get to the quarterback before he can get the ball away. Offenses generally respond to this threat by assigning extra blockers to the pass rushing tandem, trying to chip them away from the pocket to give the quarterback additional time.
In an effort to thwart the effectiveness of this tactic, and possibly punish the offense for over compensating to protect the outside, Larry Coyer will send the defensive tackles in front of the defensive ends, to hit the offensive tackles, and send the defensive ends around the defensive tackles to the inside of the offensive line. This movement, which takes place after the ball is snapped, is called "stunting."
The idea behind this procedure is to take advantage of the offensive line assignments. The offensive tackles are planning on quickly dropping back to force the defensive ends away from the pocket. The tight end or running back is assigned to help chip the pass rushers as they come around the outside of the offensive line. The center and guards are assigned to the defensive tackles or any player who may blitz (usually safeties or linebackers). Sometimes a guard will be instructed to help with one of the defensive ends.
If all of the offensive linemen simply do their "jobs," a hole will open up briefly in the heart of the offense. If the defensive ends can move quickly enough, and the offensive line does not react and recover fast enough, the stunting defensive end will come unblocked right up the middle, in the face of the quarterback.
The concepts behind the responsibilities of defensive ends are not that hard to grasp. Typically, offensive linemen, offensive line coaches, and offensive coordinators know the options defensive ends have to wreak havoc in their plans. It takes talented players, cohesion between defensive ends and defensive tackles, and a lot of practice to do it right. The Colts generally have those things, and that is why they are one of the most feared pass rushing teams in the NFL.