I don’t always understand the reset

My understanding has always been that the chains had to be set before the ref could signal for the clock to start. But, in recent years, I’ve noticed a few times where the ref signals for the clock to start while it looks like the chain crew is still moving into position.

I’m not sure if it was a rule change. Or if the chain crews have gotten better/quicker. And, it’s possible that the guy I saw running into position was the down stick guy who I think is to stay in position at the old line of scrimmage until it is certain there were no penalties on the previous play.