If you are doing it and it simulates the snap, the penalty would be on the offense. He would have to step back slowly from center, similar to a shift (SLOWLY) and it still might be on the the offense if the defense jumps.
Honestly, if you are just doing it to try to get the defense to encroach it is most likely to be on the offense.
QB can go in motion from the shotgun no problem. (still can't simulate the snap)