A stunt in football is a maneuver in which two or more defensive players quickly rush the offensive line after the snap in order to disrupt the play. Rather than simply take on offensive linemen one-on-one, stunts involve trickery (i.e. two defensive linemen looping around one another) to create spacing and make blocking assignments more difficult for the offense to execute.
Stunting is an incredibly effective way to confuse the offensive line on passing plays. Fakes, juke/spin moves and switching places right as the ball is snapped all make it easier to get to the quarterback by creating holes in the offensive line that can be exploited.
Performing stunts on third downs (generally obvious passing downs) is especially beneficial to the defense, as it forces the quarterback to release the ball before his receivers can get open for a pass. This makes it difficult for the offense to obtain the yardage required to keep possession of the ball and increases the chances of the quarterback throwing an interception.
Stunts are generally not effective against run plays because they take time to develop. That extra split second can be all the running back needs to break through the line, setting up big gains due to the lack of defenders that remain in good position to make a tackle. Thus, defenses generally do not call for stunts if they expect the offense to run the ball.
By running the same stunt multiple times throughout the course of a game, the defense runs the risk of the offensive making adjustments. Once the offense recognizes a particular stunt, it becomes relatively easy for the quarterback to change the call to a different play that is designed to beat the defender's movements.
The two most common types of stunts are listed below: