American Football NCAA Rule 9-3-5
A block in the back (other than against the ball carrier) is illegal (A.R. 9-3-3-I, VII and -IX; and A.R. 10-2-2-XII).
Exceptions:
1. Offensive players who are on the line of scrimmage at the snap within the blocking zone (Rule 2-3-6) may legally block in the back in the blocking zone, subject to the following restrictions:
(a) A player on the line of scrimmage within this blocking zone may not leave the zone, return and block in the back.
(b) The blocking zone disintegrates when the ball leaves the zone (Rule 2-3-6).
2. When a player turns their back to a potential blocker who has committed themselves in intent and direction or movement.
3. When a player attempts to reach a runner or legally attempts to recover or catch a fumble, a backward pass, a kick or a touched forward pass, they may push an opponent in the back above the waist (Rule 9-1-5 Exception 3).
4. When the opponent turns their back to the blocker under Rule 9-3-3-a-1-(b).
5. When an eligible player behind the neutral zone pushes an opponent in the back above the waist to get to a forward pass (Rule 9-1-5 Exception 4).
PENALTY—10 yards. Penalties for Team A fouls behind the neutral zone are enforced from the previous spot. Safety if the foul occurs behind Team A’s goal line. [S43].