What is the football offside trap?

Answer 1

An offside trap is when the second-last defender, seeing an attacker just barely onside, moves quickly upfield just before the attacker's teammate passes the ball to him. As long as the move is made just prior to the pass, the attacker will find himself in an offside position and if he becomes involved in play, will be called offside without realizing he was in that position.

The offside trap is an advanced tactic can doesn't always work. It relies on the attacker being unaware of the defender's movements, and having the assistant referee in the correct position to make the call. If the defender realizes what is about to happen, he may remove himself from the play and let an onside teammate play the ball, or may himself get onside before the pass is made. This leaves the defender out of position to defend against the attack, and often leads to a one-on-one play for the goal.