A punt rusher in football is a player tasked with getting past the opposition and blocking a punt attempt before the kick can be made successfully. Punt rushers are generally aligned with the outside shoulder of the furthest blocker.
Since punt rushers require a unique blend of speed and strength to get around blockers, teams often elect to use defensive ends, linebackers, and cornerbacks as punt rushers. Defensive ends and linebackers are usually able to make a seamless transition to the role, as they have plenty of experience chasing after the quarterback on passing plays. Cornerbacks best attribute is their familiarity with using their hands to knock down passes and intercept the ball when playing coverage against a receiver. This gives them a greater chance of knocking down the ball as it comes off the punter's foot.
When a member of the defense manages to get around the offensive line and swat the ball down before it can be kicked into the air, the ball is considered "live" and the defense has a chance to recover it. If a defender simply manages to fall on the ball, a change of possession occurs and the recovering team's offense takes over at the spot of recovery. If a defender manages to pick up the ball, he has the opportunity to run with the ball back to the opposing team's end zone for a touchdown.
Should the offense recover a loose ball, they have the opportunity to extend the play and try to gain a first down by passing or running the ball. In most cases, however, a member of the offense is forced to fall on the ball to prevent the defense from scooping and scoring, at which point a change of possession occurs.