Under Part 4, Rule 4.8 A: Interpretation No. 12

The legal position a player can assume when the opponent is hitting the ball from behind that player is at an angle to the left or right side of the ball being hit, as long as that position is assumed before the opponent begins his stroke.

A player must move out of the way of his opponent even though it may mean moving to an undesirable court position. It is no excuse that the player claims he did not know where his opponent was. He must move. This is also the case when a player gives his opponent a back-wall setup. It is the obligation of the first player to look over his shoulder and, if necessary, move out of the way. A player cannot line up directly in front of his opponent's back-wall shot. If a player assumes such a position and there is a collision, or if the player is struck by the ball, the hinder is avoidable.