I have been on both sides of this penalty, trying to determine the steadfast rules. When I was on the defensive side and also as a rookie referee, I was under the impression there is an imaginary plane (like the end zone) that can not be crossed prior to release. I was overruled as a referee and out argued by others that this was not the case.
I have been told as long as a portion of the player is at or behind the LOS at beginning of throwing motion, then not a penalty. Just trying to determine exact rules so that I can become a better fan.