American Football NCAA Rule 9-1-5

There shall be no clipping (Rule 2-5).
Exceptions:
1. Offensive players who are on the line of scrimmage at the snap within the blocking zone (Rule 2-3-6) legally may clip in the blocking zone, subject to the following restrictions:.
(a) A player in the blocking zone may not block an opponent with the force of the initial contact from behind and at or below the knee.
(b) A player on the line of scrimmage within the blocking zone may not leave the zone and return and legally clip.
(c) No player may violate Rule 9-1-6 (Blocking Below the Waist).
Note: 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 themself in intent and direction or movement.
3. When a player attempts to reach a runner or attempts to legally recover or catch a fumble, a backward pass, a kick or a touched forward pass, that player may push an opponent below the waist at or to the buttocks (Rule 9-3-5, Exception 3).
4. When an eligible player behind the neutral zone pushes an opponent below the waist at or to the buttocks to get to a forward pass (Rule 9-3-5, Exception 5).
5. Clipping is allowed against the runner.