Not a simple rule ...
http://www.scout.com/college/texas-tech/story/1097350-ncaa-rule-changes-part-3
Goal Line Plane Extension changed.
Now the Goal line plane runs between the pylons, and includes the entire pylon. The plane no longer exists beyond the pylons except in two specific cases: (a) When a ball carrier touches the pylon, and (b) when the ball carrier touches the ground in the end zone.
Examples: Play #1: The ball carrier dives for the corner of the end zone, leaping at the one yard line. The ball - in his possession: (a) touches the pylon; (b) goes over the top of the pylon; or (c) crosses the goal line inside the pylon. The player then first touches the ground three yards beyond the goal line out of bounds. Ruling: Touchdown in all three instances. The ball broke the plane of the goal line in the player's possession.
Play #2: The player heads for the right pylon of the goal line. At the two yard line he dives or is blocked into the air by an opponent. The ball – in his right hand – crosses the sideline at the one yd/line and passes outside the pylon. Then the runner while air born (a) touches the pylon with his foot or left hand; or (b) first touches the ground three yards beyond the goal line out of bounds. Ruling: In (a), Touchdown, since the plane is extended because the player touched the pylon. In (b), the plane is NOT extended, because the player did not touch the pylon or the ground in the end zone. The ball is out of bounds at the one yd/line. No Touchdown.
Play #3: The ball carrier heads for the right pylon of the goal line. The ball in his right hand crosses the extension of the goal line outside (to the right of) the pylon. The runner is (a) deemed to have stepped on the goal line; or (b) deemed to have stepped on the side line inches from the goal line. Ruling: Touchdown in (a) since the plane is extended by touching the ground in the end zone. In (b) there is no touchdown because the player did not touch the pylon nor did he touch the ground in the end zone. The goal line is NOT extended. The ball is ruled out of bounds at the point of crossing the side line.
Play #4: The ball carrier heads for the right pylon of the goal line with the ball in his right hand. His foot hits the pylon just before the ball (a) crosses the pylon; or (b) crosses the extension of the goal line outside the pylon. Ruling: (a) and (b) No touchdown in either case. Because the pylon is out of bounds, the ball is dead once the runner's foot hits the pylon. Thus the ball is dead in both cases before it crosses the goal line or the side line.
Play #5: The ball carrier is hit and his forward progress is stopped inbounds near the goal line at the side line to his right. When he is stopped, the ball is in his right hand extended beyond the goal line (a) inside the pylon; or (b) outside the pylon. Ruling: In (a) the ball extended inside the pylon across the goal line creates a touchdown and the ball is dead at that point. In (b) no part of his body touched the pylon or the ground in the end zone. In this case the plane is NOT extended for a touchdown. The ball is dead short of the goal line.