An offensive strategy in which a player spread out wide near the sideline will run back towards the ball at the snap in order to seal off a defender from the wide side and open up the field for a runner. A crackback block is illegal if contact is made below the waist, and can be assessed a fifteen yard penalty. Crackback blocks are most useful on plays meant to get around the edge of the defense and gain yardage.
Crackback blocks are most often used by wide receivers or tight ends split wide on plays where the offense wants to get around the end of the defense. In a good crackback block, the offensive player will block the defender back towards the rest of the defense and open a hole on the outside of the field. Players attempting to make a crackback block need to be cautious, since the player will usually be moving another direction and a low block could result in a severe penalty.