Offside trap
The offside trap is a defensive tactic designed to "trap" the attacking team into an offside position. When an attacking player is making a run up the field with a team-mate ready to kick the ball up to him, all the defenders (except the goalkeeper) will move up-field in a relatively straight line in order to put the attacker behind them just before the ball is kicked, hence putting the attacker in an offside position when the ball is kicked. Defenders using this tactic often attempt to bring an attacker's potential offside status to the attention of the assistant referee, typically by shouting or raising their arm.
The use of the trap can be a risky strategy as all the defenders (except the goalkeeper) have to move up together in a relatively straight line, otherwise the attacking players will not be in an offside position as long as they are behind the goalkeeper and a defender that has not moved up; if the offside trap fails, the attacking players will have an almost clear run towards the goal. The 2003 rule changes have made it even more perilous as a tactic, since the definition of active play was made more stringent. Thus, teams attempting an offside trap are less likely to have an offside offence called when they have caught a player in an offside position if he is deemed by the referee to be not in active play.
There ya go.
