The attacking team is awarded a corner kick. Player A from the attacking team lines up to take the corner kick. Player B from the attacking team leaves the field of play – about three yards past the touch line. Player A taps the ball from the corner kick and Player B runs from his position off the field of play onto the field of play and kicks the ball towards the goal.
Has Player B infringed on the laws of the game by leaving the field of play without the permission of the center referee and then played the ball. Leaving the field of play has allowed Player B to develop more speed at the point he plays the ball from Player A and perhaps also involves some trickery as the other team might not have accounted for this player to defend.
USSF answer (February 5, 2010):
Players are permitted to leave the field without the referee’s permission (1) during the course of play to avoid an obstacle (opponent, teammate, referee), (2) to retrieve the ball when it has left the field, (3) to put the ball back into play at a throw-in or a kick restart, (4) to signal that they are not involved in play during a possible offside situation.
Players are not allowed to leave the field of play simply to station themselves conveniently for a restart being taken by another player. As we said in an earlier response, with the exceptions noted above, players are expected to be and remain on the field of play. Leaving under the circumstances described would NOT be in the course of play and, if the referee decided that it was being done for unfair tactical reason, the action would be cautionable.