Illegal shifts that convert to a false start (ILS-FST) are pretty easy when there has been a huddle (whether all 11 gather closely together, or somewhat spread out, awaiting instructions from the sideline) and then they move somewhat quickly to their final positions, but there is one guy that hasn't quite got set when the ball is snapped very soon after the other 10 have stopped.
But, what happens occasionally is a team moves from their "huddle" to their final positions, and 10 players stop in their positions for 3-4 seconds, while one 'straggler' - usually a wide receiver - is still trying to find his perfect spot, and, he still doesn't get stopped completely before the ball is snapped. The optics of that is kind of weird for a false start, but that is, indeed, an illegal shift that converts to a false start, and is a dead-ball foul.
If they move from a tight huddle, a 'muddle' huddle, or a loose formation into final positions, the ILS-FST rule applies. This includes those situations when they are in their general positions, but are looking to the sideline for the play call, then, after the play call, interior linemen turn and set into position, and the backs and WRs adjust to their final positions. All 11 must get set for 1 full second thereafter before the snap. If not, that's a dead-ball ILS-FST.
But, once all 11 get fully stopped in a football formation (not a tight huddle, a 'muddle' huddle, or a loose formation) for one second after the ball is ready, then the ILS-FST rule doesn't apply. After that, if a couple of guys (or more) move simultaneously (and not a false start), that's a shift, and the entire team has to stop, again, for one full second before the snap. If they don't, that's a live-ball illegal shift foul. If one guy starts smoothly, but is moving forward at the snap, or came from the line and then the ball is snapped, that is live-ball illegal motion.
Also note that, if all 11 are fully stopped at the moment the ball is declared ready for play, they have met the 'stop' requirement, and do not have to wait one more full second to snap the ball. They may snap immediately upon the ball being declared ready. You'll see that in 'last second' situations, when Team A needs to get the ball snapped before the clock expires. Say the clock is stopped momentarily for a first down, and to get a replacement ball into the game, Team A may have time to get all 11 in formation and stopped before the U/C gets the ball down, steps away, and the ball is declared ready (and clock re-started). They may snap immediately.
