Rule 7 – Section 1 – Article 2 – Shift and False Start
b. False Start. Each of the following is a false start by Team A if it occurs prior to the snap after the ball is ready for play and all players are in scrimmage formation:
(a) It is not a false start if a Team A lineman immediately reacts when threatened by a Team B player in the neutral zone (Rule 7-1-5-a-2) (A.R. 7-1-3:V)
(b) It is not a false start if the snapper takes his hand(s) off the ball, provided this does not simulate the start of a play (Rule 7-1-3-a-3).