The rule to my understanding is a player must be set for 1sec prior to a snap, and only one player can be in motion at the time of the snap. The Lions ran that 6 man front on a few occasions Sunday. this was the front that the Patriots really hurt the Rams during the SuperBowl. McVay has the RB step up in the A-gap to account for the extra opposing player when presented with this front. So in your example as long as Henderson is set for 1second another player could motion legally when the ball is snapped.
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XXXIVwin
I believe because Henderson always comes to a complete stop prior to the snap.
