A defensive player is allowed past the first line of the offense only to be blocked in the backfield by an additional offensive player.
Sometimes referred to as a "mousetrap"
In trap blocks a single or multiple defensive players are allowed past the offensive line in order to be blocked in the backfield, allowing for additional running or passing routes past the line of scrimmage. The defensive players are then essentially behind the play, which opens up more field space for running backs and receivers. There is more potential for holding penalties on the offense, however, as defensive players try to get back into the field of play.