If a quarterback is allowed to sit in the pocket and look for receivers to throw to, eventually he'll find somebody open. For this reason, defenses will do what is known as "blitzing" to create pressure on the quarterback. The more pressure a defense can put on a quarterback, the better the chances are that he'll throw an incompletion, an interception, or not get a chance to throw at all.
BASICS OF THE BLITZ
Those only peripherally familiar with football may not understand the purpose of the blitz. After all, isn't the defense trying to get the quarterback on every play? Well, yes. The defense will have three or four defensive lineman rushing the quarterback, depending on their alignment. The linemen are up against five offensive linemen, and optionally a tight end and a running back. Take a look at a traditional offensive and defensive formation.
LB LB LB
CB DE DT DT DE CB
WR T G C G T WR
WR QB TE
Examine this from the defensive side. There are two cornerbacks on the outside receivers. Either the free safety or a linebacker will have to cover the slot receiver. Another linebacker or defensive end will have to cover the tight end if he doesn't stay in and block, and the running back must also be accounted for. The folks who are trying to get to the quarterback don't have any advantage against the folks trying to stop them. In fact, they're at a disadvantage, because they don't know where the ball is going.
That where the blitz comes in.
Here's another formation, except that this time the defense is in what's known as a "nickel package". They're a little better equipped to deal with three wide receivers.
/ LB LB
CB CB* DE|DT DT DE CB
WR \ T G C G T WR
WR \ QB TE
Here, a fifth defensive back (the cornerback marked with the asterisk) is on the field in place of a linebacker, and lines up across from the slot receiver. As the offense lines up, the free safety takes a few steps towards that nickelback. When the ball is snapped, the designated cornerback blitzes -- that is, he makes a bee-line for the quarterback. The free safety moves up and is responsible for covering the slot receiver. The defensive end engages the tackle and the linebacker engages the guard. With the left side of the line engaged in blocking, the running back is responsible for picking up the cornerback. If the running back goes out into coverage, or misses the blitz pickup... as Theo said in "Die Hard"... "Oh my God, the quarterback is toast!"
Although the defense sent only one more player than usual, it was a guy that the offense didn't expect and didn't plan for. This is the idea behind the blitz.
BEATING THE BLITZ
The object of the blitz is to drop the quarterback quickly. Therefore, the best way to defeat the blitz is for the quarterback to get rid of the football. Running the ball consistently is a good way to slow down the blitz, but often times a team doesn't have a rushing attack worth respect, so the defense keys on the pass. The offense can substitute a better blocking back to help pick up the blitz, or use additional tight ends. The most effective way to beat the blitz, though, is by calling good plays, some of which are detailed below (the defensive side has been removed for clarity).
The quick slant
WR / OL OL OL OL OL WR
WR QB TE
On a quick slant, the wide receiver runs immediately into the area vacated by the blitzers. The quarterback takes a three step drop and then immediately steps and throws into the middle. As a side note, notice that the slot receiver and the tight end are not on the line of scrimmage. These players signify the end of the offensive line, and they and anyone else outside of them on the field of play are eligible receivers -- that is, they are allowed to run downfield. Notice also that the outside wide receivers are on the line of scrimmage. This is because there must be at least seven men on the line of scrimmage when the ball is snapped. Just some additional football info you may not have known. Back to beating the blitz...
The screen pass
WR | OL OL OL OL OL WR
WR QB TE
The screen pass takes advantage of the aggressiveness of the defense. On a screen, the linemen will actually let the defense past them, making it look as though they've been beaten to the inside. At the same time, the running back runs out into the flat, and takes a short pass from the quarterback. The wide receivers and offensive linemen then focus on blocking downfield for the running back, ignoring the defensemen who have rushed themselves out of the play.
WR OL OL OL|OL OL WR
WR \ TE
The draw is another play that takes advantage of the defense's overpursuit. Drawn up here in the shotgun, the ball is handed off to the running back, who runs straight up the middle. The offensive line must deflect the oncoming rush away from the hole that the running back is going to run through. This play can also be run by the quarterback (appropriately called the 'quarterback draw'), and the play can also be run as a delay. That is, the quarterback can hold on to the ball for an extra count to allow the defense further upfield, and then hand off.
THE INTELLIGENT QUARTERBACK
Defeating the blitz through play-calling requires that the offense has actually figured out that the defense is blitzing. When it's third-and-seventeen, it's a pretty good bet. When the defense has blitzed ten times in a row, it's a pretty good bet. And when linebackers have walked up into the gaps and are twitching uncontrollably, it's a pretty good bet. Hopefully, the quarterback can read a blitz ahead of time, so that if the offensive coordinator hasn't anticipated the blitz, he can audible into a suitable play.
For that reason the defense must do their best to disguise the blitz, or send blitzers that the offense wouldn't expect. Send a cornerback or a safety, from the strong side or the weak side. Varying the defense with different formations, using different blitzers, and blitzing from different sides are all ways to effectively blitz. The idea is to create confusion. If the quarterback doesn't know what's coming, he can't react.
Let's assume that the quarterback *DOES* know what's coming. The defense has called a corner blitz, just like above. Mr. QB takes a look and calls an audible. The running back puts the blitzer on his back, and the quarterback has time. Now there are three wide receivers and tight end in single coverage, covered by two corners and two safeties. When you consider that defensive backs can maintain coverage for about three-and-a-half seconds at best, there's trouble. A quarterback with time to throw will pick apart man-to-man coverage all day long.
HE AIN'T SO SMART - THE ZONE BLITZ
Let's face it... that pretty-boy signal caller is making your rough and tumble defense look bad. He's picking up your blitzes and his receivers are leaving your backs eating grass. Don't worry about it. That's why they invented the zone blitz.
The zone blitz was invented sometime in the late 70's, but didn't really become a factor until the mid-90's, when the Dom Capers-led Carolina Panthers reached the NFC Championship in just their second year of existence. Most of their success was due to a suffocating zone blitz that left quarterbacks on their asses and point totals in single digits.
The zone blitz utilizes -- guess what -- zone coverage rather than man-to-man. Each player is responsible for covering a specific area of the field, and on the snap of the ball, some players rush the quarterback and some players drop into coverage. The combination of rushers and coverers changes with each blitz call. A defensive lineman may cover short middle. Or a linebacker. Or a safety. Or anyone. The point is, the quarterback never knows who is covering which part of the field, and the blockers never know where the rush is coming from. Normally a quarterback can find holes in zone coverage fairly easily, and can still do it against the zone blitz. It's just much more difficult -- the initial disguise prevents the QB from prereading the holes in the zone, and the increased pressure stops him from finding the soft areas of the defense quickly enough.
The zone blitz is just the latest innovation in blitzing. As offenses devise ways to defeat blitzing schemes, defenses come up with better, more clever ways to attack the quarterback. The intricate game of attack and counterattack continues...
key to diagrams
CB = cornerback; FS = free safety; SS = strong safety; LB = linebacker; DE = defensive end; DT = defensive tackle; QB = quarterback; WR = wide receiver; TE = tight end; RB = running back; OL = offensive lineman; G = guard; T = tackle; C = center;