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In the fourth quarter when there is only a few minutes left in the game and one team is winning by 17 or more points, it's common in the NFL for that team to go into a "prevent" defense. In a prevent defense the idea is to make the other team use up a lot of time. The clock is stopped when the player with the ball steps out of bounds, so the first goal of the defense is to keep everyone in bounds. The only danger to the defense is giving up a big play, for example a 25 yard or longer pass or run. It doesn't matter to the defense if the other team makes a lot of plays, and gains four to 8 yards per play, as long as the clock keeps running and time keeps ticking down.
In this case the defense will pull back into what is called a soft zone. They will most likely play in a nickle, dime, or quarter package. A soft zone means that all the safeties and cornerbacks are playing back, five or ten yards off the line. The free safety will often play as much as 20 yards back. There will be no jamming of receivers on the line. The zone means that each defensive back is responsible for an area of the field, so they're all watching the quarterback's eyes instead of running stride for stride with a receiver. It is very easy for the offense to make short plays against this defense, gaining four to eight yards per play, but it's almost impossible for the offense to make a big play of 20 or more yards against this sort of defense.
Many fans hate the prevent defense, saying that it prevents the win. While it seems logical to think that it doesn't matter if it's your offense or their offense that eats up the clock, there is another argument. Your normal defense apparently was very successful at stopping their offense and getting you to a place where the game is all but won. Why change what is working in the last minutes? Still, one frequently sees the prevent defense hauled out in the last few minutes of a game that is not close.
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It's difficult to get good rushing ends in the NFL, and far easier to get good linebackers. This has led some teams to use a 3-4 alignment instead of a 4-3. What this means is that instead of using four defensive linemen and three linebackers, they use three defensive linemen and four linebackers. In the 3-4 defense, normally the four linebackers are the key to the defense, so the three defensive linemen are playing a 2-gap scheme, that is they are trying to occupy the offensive linemen and keep their linebacker free to roam. In the 3-4 the linebackers are called inside and outside linebackers.
Since it is most often linebackers that stop running backs, the 3-4 defense is normally especially good against the run. However, since there are only three linemen to try to collapse the pocket and rush the passer, the 3-4 defense has struggled against the pass compared to the 4-3. To compensate for this several teams that use the 3-4 invented a technique called the zone blitz. In this technique, the nose tackle, one of the two other tackles, an inside linebacker, an outside linebacker, and often a safety will all rush at one side of the line, so there are five players rushing in on three offensive linemen. This would normally leave a big hole in coverage, however in the zone blitz the remaining two linebackers will shift over and drop back into a cover-2 type defense, each defending half the middle of the field, and the remaining tackle will drop into coverage and try to cover an outlet receiver, like a tight end or fullback. After several years of working out details of responsibilities, the 3-4 with a zone blitz package has proven quite effective. The New England Patriots used a 3-4 defense to win three of the first four superbowls in the 2000s.
In the 90s some of the charming aspects of the 3-4 defense were that few teams played it, so the offenses were unaccustomed to playing against it. The 3-4 frequently had big elements of surprise. Also, on draft day the teams running the 3-4 were trying to draft different sorts of defensive linemen and linebackers than other teams, so they found there was less competition for their type of players and they had a lot of success getting who they wanted. However, the NFL is very much a copy cat league, and now that the 3-4 has proven to be very successful in the playoffs and superbowl, everybody wants one. It's now (2005) getting easier to draft players for a more traditional 4-3 due to the large number of teams trying to change over to the 3-4.
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When the offense has the ball on the one or two yard line, they will most often try to run the ball directly into the endzone. To prevent this, the defense will sometimes use a "goal line" defense, where there are seven down linemen on the field. They will be backed up by some combination of linebackers and safeties, depending on the physical skills of the particular players on this team. The object of this defense is to stop the run at pretty much all costs.
A solid passing team will counter this defense with a play action fake: the Quarterback will fake a handoff to the running back, and the fullback and offensive linemen will all act as if the running back has the ball and they have to make a hole for him. Meanwhile, one of the receivers will try to sneak into the endzone, and the quarterback will try to throw a quick pass to him while the defense is all tied up stopping the fake run.