Rule 9 – Section 3 – Article 6 – Blocking in the Back

A block in the back (other than against the ball carrier) is illegal. (A.R. 9-3-3:I, VII and IX) (A.R. 10-2-2:XII)
Exceptions:

3. When a player attempts to reach a ball carrier or simulated ball carrier or legally attempts to recover or catch a fumble, a backward pass, a kick or a touched forward pass, he may push an opponent in the back above the waist (Rule 9-1-5 Exception 3).