What it is

A trap is a run where a defensive lineman is allowed to cross the line, and a pulling lineman from the other side then blocks him from the side.

In a game

The defender thinks he has beaten the blocker and rushes forward, but the guard who has pulled across hits him from a new angle and opens a hole behind him.

More detail

The play punishes aggressive defensive linemen who always attack upfield. It works best when the defense has been rushing hard against earlier passes, because then the defenders are eager to charge into the backfield.

Animated diagram

1. The line blocks the defensive tackle on the right side by letting him go across the line.
2. The tackle charges forward, thinking he has beaten his blocker.
3. The guard from the other side runs across and hits him from the side, and the back runs through the hole.