There is a well documented thing called the Tetris Effect, where you see the blocks in dreams or when your eyes close. There's been some neuro research on it. When I asked these guys about it, they said they got past it after a while. They all say, yes, I have experienced Tetris Effect stuff, but at some point the brain seems to filter it out.

My impression is that these players are spending a lot of their conscious energy on the next-piece box, constructing the stack, and just staying calm. I think the majority of what they're doing is near instinct at this point, since they play so much, they just kinda "know" the right move for any given situation. Or at least, most of them. A max out game is a combination of random luck and extreme skill/practice.

The documentary "Ecstasy of Order: The Tetris Masters" (steaming free on Hulu) has more on all of this. Disclosure: I wrote parts of that film. Ben Mullen is one of the main subjects of the film.