The purpose of this drill is to teach a player to focus on going when the ball moves. This is a great drill to teach defensive linemen to concentrate on the ball and to ignore the quarterback's cadence.
You can line up multiple players on the defensive line side of the ball. Players should be in a 3 or 4 point stance. Try to get players to jump off sides by using different snap counts and cadence.
Players should only jump ("get off") when the ball is snapped.
This is a great way to build concentration in defensive linemen.