American Football NCAA Rule 9-1-6
a. Team A prior to a change of team possession:
1. Linemen with initial position completely inside the tackle box may legally block below the waist inside the tackle box on their initial line charge. A block initiated 1-yard beyond the neutral zone is considered within the tackle box. After the initial line charge, these linemen may block below the waist within the tackle box until the ball leaves the tackle box only if the force of the initial contact is directed from the front.
2. Stationary Backs lined up with any part of their body inside the tackle box may block below the waist within the tackle box until the ball leaves the tackle box only if the force of the initial contact is directed from the front. “Directed from the front” is defined as within the clock face region between “10 o’clock and 2 o’clock” forward of the area of concentration of the player being blocked.
3. All other Team A players are not allowed to block below the waist.
b. Team B prior to a change of team possession:
1. Players aligned in a stationary position within 1-yard of the line of scrimmage within the tackle box may legally block below the waist in the tackle box on their initial line charge.
2. All other Team B players are not allowed to block below the waist except against a runner.
c. Kicks. During a down in which there is a free kick or scrimmage kick, blocking below the waist by any player is illegal except against a ball carrier.
d. After change of team possession. After any change of team possession, blocking below the waist by any player is illegal except against a ball carrier.
e. Clipping. No player may violate Rule 9-1-5 (Clipping).