The eleventh rule of soccer is probably the most difficult to grasp among novel fanatics. It's called the "offside rule" and it can invalidate a play due to a specific postion of the teams inside the field.
The offside position can only be taken by an offending team (a team in control of the ball) and happens under two fundamental circumstances:
1) An offensive pass takes place. Also known as "a forward touch of the ball", an offensive pass is one that moves away from the offending team's goal and towards the defending team's goal. Lateral and backwards passing cannot be subject to an offside call. Throw-in passing is also exempt.
2) Potential pass receivers are in the rival half of the field.
So what's needed for a player to be caught in an offside position?
In a broad sense, at the moment of the offensive touch, there must be at least two rival players between any potential reciever and the rival's goal line for him or her to be able to participate in the play. More technically, a player is considered to be offside when: (1) at the same instant that his or her teammate touches the ball, (2) any of the player's body parts that would be allowed to touch the ball at any other time of the play are (3) closer to the opponents' goal line than (4) both the ball and the second-to-last opponent. Hence, the arms of a player cannot be considered to be offside.
An offside position is not a game stopper per-sé. The offside rule states that, in order to call an offside offence, the player(s) must start active participation in the play from an offside position. Active participation can be touching, running or jumping towards the ball, blocking a defender from reaching it, or any other action that derives in advantage; like recieving a deflected ball or a simple distraction of rival players. In that case, the assistant referee must raise his flag to inform the central referee who, due to the nature of the play, generally lacks the perspective to make a proper judgement.
Once an offside offence has been called by the referee, any action (goals, mostly) that takes place afterwards is nullified. The ball possesion is reversed to the affected team and the game is resumed with an indirect free-kick.
Since the two-opponents variant of the rule was defined in 1925, it has played an important role in strategy and tactics, both offensive and defensively. Some center-forwards use it to sneak behind the back of defenders to an offside position and then, unexpectedly run into the play to "pull" defenders away from, or to serve as a link between charging players. Defensively, the infamous offside-trap was a most effective resource during the twentieth century, though it had the nasty effect of pouring cold water not only on the most eager attackers, but also on most supporters' spirits.
Nowadays, the offside-trap is core material for every defender's basic training drills, although gone are the days when a team's overall strategy could be based on it. In a scene where fast-paced courts, first touch plays and sweeper-goalkeepers are the trend, depending on the offside trap is just too risky, for it means giving away ball posession and wait for an opponent's misjudgement; a chance that less and less coaches are willing to take.
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