It changes based on timing, situation, my own emotional state (fear and nervousness can make me either very stiff or very stimmy, not sure what makes the difference), etc. Sometimes I can go a long time without stimming, sometimes I do it all the time.
And then which one depends on a whole lot of things, including why I'm doing it. Why can be it's an automatic movement in the background that probably has a purpose but I don't know what. It can be a defense against motion sickness. It can be a tic (not really the same thing, but can look the same). It can be an expression of the sort of excess energy being burned off from an emotion. It can be to force body awareness. It can be a way of becoming physically involved in comprehension of my surroundings. It can be a way of relaxing. It can be just because it feels good. It can be a way of managing overload without blowing up or shutting down. If it is that, it might be a deliberate thing ("this made me feel better last time, so...") or it might be an instinctual reaction somewhere deep in my brain. It can be part of shutting down, though, too. It can be a way of dealing with stress. And modifying the motion in some way can be a means of communicating certain things.
...And all those things get called the same thing.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams