Presence/nonpresence Transition

[Ozzy]

Today I was thinking about this question: what exactly brings you back to awareness/presence during meditation when you lose it for a while (mind wandering). I probably experienced this moment thousands of times already but never wondered how does that happen that one second you are "asleep" and then the next second you snap back to being present. Is it the fact that I simply trained my mind to do it by repeating the process so many times? Still, I would like to know the mechanism by which it happens.