Muscle memory is the phenomena that occurs when our minds finally notice that "hey, I've got deja vu", and this happens after dozens or hundreds of repetition of the same event.
We are able to drive our car, or walk and get somewhere and not even realize we were walking/driving.
It enable athletes to react to stimuli and do the correct thing rather than having to think about it and screwing it up.
With photographers, after years of experience, it is possible to "know" when to trip the shutter at the right time.
That doesn't mean we aren't thinking but that we aren't consciously thinking.
Of course some photographers never get the exact millisecond right, but that's just because they suck. ( I mean that in the most diplomatic way)