Prevent defense

Prevent defense is an American football defensive alignment that uses seven or more defensive backs or players in a defensive back role, the goal of which is to prevent the opposing offense from completing a long pass. This defense is frequently used in obvious passing situations, such as a third-and-very-long situation, or to prevent a long score on the last play of a half, or when the defense believes that the offense must pass (for example, if the offense is trailing late in a game). Otherwise the prevent is almost never used. The defense trades size for speed, and tries to ensure that no receiver can get behind the defense. A prevent defense backs up the coverage so far that it will often give up long yardage, but it makes scoring a touchdown in a single play very difficult.

Quarter defense
The quarter defense has three down linemen, one linebacker, and seven defensive backs. The quarter defense gets its name as the next coin after nickel and dime in the sequence of defenses. Quarter or quarters can also be used to describe a type of zone pass coverage, in which four defensive backs divide the field into vertical quarters for zone coverage. This coverage may be combined with a 4-3 or 3-4, and is used to take away deep routes but maintain a good pass rush and run coverage.

Half dollar defense
Half dollar defenses have eight defensive backs and are very rare and are often used to prevent a Hail Mary pass. Professional teams may not have enough defensive backs on the roster to play the quarter or half dollar, so wide receivers are sometimes used to fill the extra positions, particularly in late game situations where the receiver's offensive skills can be put to good defensive use.

Implementation
In this case the defense will pull back into what is called a soft zone. They will most likely play in a nickel, dime, or quarter package. A soft zone means that all the safeties and cornerbacks are playing back, five or ten yards off the line of scrimmage. 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.

Advantages
In the fourth quarter when there are only a few minutes left in the game and one team is winning by 7 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 eight yards per play, as long as the clock keeps running.

Examples
In Super Bowl XXXII, Denver Broncos coach Mike Shanahan famously instructed his defensive coordinators to keep playing the same defense as the Green Bay Packers attempted to drive downfield in the final two minutes of the game. The Denver defense managed to stop the Packers' drive, which led to the Broncos' 31-24 victory.

The Denver Broncos famously botched the prevent defense in the 2012 AFC Divisional Round playoff game against the Baltimore Ravens. With less than 40 seconds to play, the Ravens needed a touchdown to tie the game and faced a 3rd down from their own 30 yard line. Broncos safety Rahim Moore allowed Baltimore receiver Jacoby Jones to get between Moore and the endzone, where Jones caught a 70-yard touchdown pass from Ravens quarterback Joe Flacco. The Baltimore Ravens went on to win the game in double overtime, and eventually the Super Bowl.

Disadvantages
The prevent defense is rarely used on consecutive downs, or with a significant amount of time remaining, since a team with time to move downfield would easily be able to gain plenty of intermediate yardage. John Madden once said, "All a prevent defense does is prevent you from winning."

The bend-but-not-break nature of the prevent defense tends to give the offensive team many easy gains but no big play, so the prevent defense can make the end of the game uninteresting for fans. Some coaches avoid using the prevent defense, choosing instead to continue playing the same defensive schemes that ostensibly gave them the lead to that point.