Well, as an example - when I was young, I had a stone jumping game. It was a variation of the common 'don't step on the ground game'. The variation was this: At each choice for a next stone to jump to stone I would sometimes think: "Perhaps I have existed before, and done this all before, exactly so. Is it possible to choose a new stone this time? Or will it always be the same stone? If it has been exactly so every time, then even this thought about wondering if it's possible to choose a new path was thought before as well". And I would jump, stone to stone, wondering if it had been done before and if the path could have been different this time or if it must have always been the same, and forever always continue to be the same. As a post-DP twist I've since added this: maybe I don't choose a PATH at all. Maybe at each possible instant, everything that could have happened, DOES happen, and rather than choose a path, I only associate my self with one of the infinite possibilities.
The uncontrollable DP that I later experienced around 18-19 was a difficult one and it sure as hell made me question even more things about the world and myself than I ever would have wanted to. Unlike some people here though I don't think it was due to trauma - I was just always thinking things like this and just eventually had perceptual experiences that very literally turned my perspective of the world and myself inside out.
I experienced it in the manner that Susto seems to describe - an intensified awareness of 'where' you are in your mind. 'Normally' you feel like you are looking out of your eyes like they are windows out into the world or something - but with DP I was intensely aware that everything was in my mind. It was the suffocating mind-in-a-box sensation, feelings of complete isolation from 'outside' (where is outside then?) and inability to describe it all or relate to others that made it all extra-terrifying. As far as how the mind works I have an interest now in psychology, neurology, physics, but ultimately this is something that no one knows. The brain though - after all this I believe it's more like a rapid transit system for conducting electromagnetic waves rather than the source of consciousness - those waves, THOSE are what is alive and 'moving around' in there like some strange cloud - the brain itself is just a bunch of cold tunnels.