One thing that is often a part of being autistic is the need to stim.
Stimming is a jargon term for a stereotypy, a repetitive body movement that self-stimulates one or more senses in a regulated manner. It is one of the symptoms listed by the DSM IV for autism, although it is observed in about 10 percent of non-autistic young children. Many autistics have no stims. Common forms of stimming among people with autism include hand flapping, body spinning or rocking, lining up or spinning toys or other objects, echolalia, perseveration, and repeating rote phrases
I'm not sure about the 10% figure for non-autistics, but people with ASD clearly stim at a much higher rate. Stims are also not limited to body movement, playing a CD or even the same song over and over can be a stim. Stimming is much more than the above article claims.
For me, stimming has a calming affect that is more than just relieving stress. Sometimes I stim because I need the distraction in order to be able to pay attention. Sometimes I'm not able to focus on one sound over the others without stimming. I have troubles when there are several sounds going on around me and I need to pay attention to just one, sometimes stimming makes it easier to focus on and separate the one sound I need from the others. And other times I have no idea that I'm stimming, or even why. When this happens it has been known to drive my fiance crazy (to her credit she loves me, quirks and all.)
My common stims are rocking back and forth, bouncing a knee (or foot tapping,) listening to a CD or song on repeat, and of course all kinds of hand flapping, tapping, wringing and fidgeting. Less common are spinning, walking back and forth or in circles, chewing on the insides of my mouth and stacking things. My son also rocks back and forth and flaps his hands and listens to music on repeat. He also chews on his fingers and clothes and feels the need to taste everything (and I mean everything.) Good news is, he does not make his fingers bleed (and I don't bleed when I chew on the inside of my mouth,) but he does do a number on his shirts. Somehow I'll figure out how to copy what my mom did to get me to stop chewing on my fingers and clothes, hopefully I do so soon (replacing shirts every month or so is expensive.)
I've learned the hard way that trying to not stim is bad. If I don't stim I become less functional. It's just a fact of life for me, so I just let it happen and no longer try to stop it when I notice. So the next time you see someone sitting down, rocking back and forth while flapping their hands, remember that it might be someone like me.