[jsarno]

Here is the article that says it all:
Article 5 At the snap, all offensive players must be stationary in their positions:
(a) without any movement of feet, head, or arms;
(b) without swaying of body; and
(c) without moving directly forward except that one player only and he, playing in a
backfield position, may be in motion provided he is moving, parallel to, obliquely
backward from, or directly backward from the line of scrimmage at snap.

It's on page 41 FYI.