http://home.comcast.net/~minnmo/foot...-newFormat.doc
an excerpt from the above article.
PLAY: Set backs A10 and A32 mistakenly both go in motion on the QB’s signal. Their motion is parallel to the line of scrimmage. The ball is snapped (a) while both are still moving, or (b) after A10 stops, realizing that something’s wrong, while A32 continues with his motion. RULING: In (a) Team A is guilty of illegal motion. They have two players moving at the snap. [Note: this is a foul that a lot of referees mistakenly call an illegal shift.] In (b) A is guilty of an illegal shift. Once A10 stops, he’s completed a shift, which requires all eleven offensive players—including A32—to be set for at least one second.