"Stimming", or self-stimulatory behavior, is generally considered to be some (often repetitive) action that helps reassure an Aspie or autistic when the individual feels overwhelmed. Typical stims include "flapping" (which can include any repetitive hand gestures - even my rhythmic knuckle-cracking, of each joint in order, counts as "flapping"), spinning around in a circle, or repeating a given phrase over and over. (Oddly, if you repeat a given phrase while sitting cross-legged, they don't call that "stimming" - suddenly it's a "mantra", and you're "meditating".)
My personal stims also include obsessive reading, and in extreme cases singing the Emerson Lake & Palmer song "Battlefield" (part IV of their "Tarkus" suite). I find the patterns of the music soothing. If things are really bad for me, I might add the last verse from the live version; if depressed, I'll even replace it with Supertramp's "Ruby" (which my family recognizes as a sign to get me the frak out of the situation before I melt down).
As noted, stims tend to soothe one when in an overwhelming situation; so far as I know, no one has yet done an intensive study of the topic. However, I think that the stims may provide a regulated flow of expected information, helping one cope when the incoming data from the outside world has become both unregulated and unexpected, overloading one's mind.
Some autistics, when severely overloaded, even engage in self-injurious behavior, often involving head-ramming. My daughter, when frustrated, has been known to run headlong into walls; my younger sister bites her own hand with sufficient power and frequency that she has a callus from it.
_________________
Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.