If the second to last defender’s momentum pushes him or her out of bounds while playing a ball, do you just presume that he or she is out of the play and you just line up with the new second to last defender (for offside) or what do you do?
USSF answer (July 2, 2010):
Any defender who leaves the field over the goal line without the permission of the referee, whether voluntarily or involuntarily, is still counted for offside purposes.
You will find this paragraph in the back of the Law book, under Law 11 in the Interpretation of the Laws of the Game and Guidelines for Referees:
Any defending player leaving the field of play for any reason without the referee’s permission shall be considered to be on his own goal line or touch line for the purposes of offside until the next stoppage in play. If the player leaves the field of play deliberately, he must be cautioned when the ball is next out of play.