A defensive strategy in which the pass rush will attempt to keep the quarterback in the pocket to avoid letting him scramble for more time to pass or run with the football. Contain is particularly important when facing quarterbacks with good running ability, since a quarterback who breaks out of the pocket will often have the defense out of position and can gain big yardage on the ground. The easiest strategy behind keeping contain is to have the defensive ends or outside linebackers bracket the quarterback in order to keep him in the pocket, and slowly shrink the pocket using the pass rush to cause pressure.
Contain is most important to defenses that are worried about a mobile quarterback doing damage with his legs. A quarterback who gets out of the pocket is much more likely to deliver a big pass or make a big run. By keeping contain, the quarterback is limited in his options on a particular play. Good defensive linemen will stay in their pass rush lanes so that they team as a unit can contain the quarterback and keep him in the pocket. A defensive player can lose contain if they are too aggressive in their pass rush and get out of position.