When Peyton Manning and other NFL quarterbacks start yelling out directions to their teams, it's not a simple matter of one player changing his mind and wanting a different play. These audibles are the product of a quarterback analyzing the alignment of his defense. Pre-snap reads can help a quarterback learn a lot about the objectives of the defense and, in turn, how the offense can best attack the opponent. Defensive alignment is the product of intense study and strategy, but the techniques used for alignments are basic and taught even to young players.
Filling Gaps
The basic goal of a defense is to cover as much of the field as possible. To a large extent, this is best approached by developing defensive alignments with the intention of filling gaps created by movement. These gaps are used as running lanes and areas for receivers to catch passes, and good coverage of the gaps can stop opponents from gaining yardage and even lead to yards lost. A football team's defense is broken down into three units, used in different ways and featuring different personnel, depending on the larger scheme. But within these units there are basic roles that are performed, regardless of the formation.
Defensive Line
The defensive line's job is the most consistent from one play to the next: Its goal is to break through the offensive line and tackle the ball handler. Defensive players maximize their odds by aiming to break through the gaps between offensive lineman -- the gaps give a defensive player more leverage against the offensive blocker. Defensive ends will try to attack from the gaps just outside the offensive line and try to evade offensive guards and tight ends. Different strategies can be used in attacking the offensive line to create imbalances on the offensive line, open up gaps and attack into the backfield.
Secondary
The secondary typically defends against the passing game and works to stop long runs. Occasionally, cornerbacks and safeties may creep up to the line of scrimmage and attempt to attack the backfield through a gap in blocking. Most cornerbacks align themselves directly across from the wide receivers they are guarding on a given play, although zone coverage may place the defensive backs in a specific region of the field, regardless of how the offense lines up.
Linebackers
Linebackers are the most versatile defenders on the field, used for the widest range of roles. Linebackers typically line up as the "second line" of defense behind a defensive line. They are spaced according to how the offense has lined up and in response to what the offense seems poised to attempt. Depending on the play call, a linebacker has several potential roles. He could cover a zone of the football field, essentially filling a gap by serving as a static defender, or man-up against an eligible receiver and attempt to prevent that player from catching the ball and gaining yards. A linebacker could also creep up alongside the defensive line and blitz into the backfield to try and sack the quarterback on a passing play, and if desired the linebacker can fake a blitz and then drop into coverage. When a linebacker does choose to blitz, it creates a gap downfield where the opposing quarterback might pass. Defenses will use different alignments to try to cover these gaps, but in some cases the gap will be left open in hopes that the quarterback doesn't see it before passing or getting sacked.
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