Sometimes the things I do are entirely mental... when I'm thinking about something, analyzing it. When my brain goes into "information processing" mode, my body doesn't really need to be doing anything, so usually it doesn't; or else I'm just stimming, usually twitching different muscles or rocking.
I also dissociate in this fashion when overstimulated, upset, in pain, or being attacked (verbally, nowadays) by anyone. Usually that sort of "zoning out" involves stepping back from my emotions, and analzying the situation rationally so that it doesn't have much emotional impact on me. The psychologists call it "intellectualization".
Oh, and when I'm being indecisive about something--when I'm not sure what to do next, when a project or physics/math problem has confused me or I don't know what needs to be done next in a task I'm doing (cleaning, etc.). That's when I kind of stand in the middle of nowhere, staring into space and trying to reorganize my thoughts so I know what comes next. It happens a lot when i'm doing schoolwork or tests, and can be improved by taking caffeine pills or drinking coffee. I really think I have non-hypreactive ADHD, or at the least my AS is mimicking ADHD...