This is always a tricky one -- and one that you have to see to really call. For a False Start you have to ask did the Full Back "simulate the start of the play". If yes, FST. If no, then illegal motion. Having said that, philosophically and practically, you should always err on the side of calling a FST and shutting the play down. More times than not, the play is coming back for the Illegal Motion, and by allowing the play to go forward you are inviting opportunities for injuries, DB fouls and other mischief. If you shut it down, you avoid all those issues; for the same reasons you try to shut down the FST that is really an illegal shift because the offense never got set, or shut it down and call illegal substitution instead of letting the play go and calling illegal participation.
