This is a defensive strategy used in football when the defense
believes the offense must pass the ball. This type of defense can
take many forms, but it typically makes use of at least seven
defensive backs, leaving only four defensive linemen. The goal of
this strategy is to "prevent" the offense from gaining any
significant yardage or scoring a touchdown in a single play, hence
the name of the strategy. This defense will allow the offense to
gain yardage within a limit that still prevents it from gaining the
yards it needs to score.
This type of defense if used most often when a team is up by at least seven points or more with little time left in the half or game.
The advantage of using the prevent defense is it enables the
defense to manage the gains the offense makes without giving up any
big plays. The disadvantage is that if enough insignificant gains
are made, the offense may actually reach the ability to score with
a single play.
There are many coaches who do not believe in using the prevent offense simply for this fact. The famous NFL coach and later broadcaster John Madden once said, "All a prevent defense does is prevent you from winning."