There's so many different and unrelated things that get called stimming it's hard to answer.
But as far as unusual/repetitive movements... that one's weird for me. It's always been the sort of thing that I would absolutely have to do at some point. There have been times in my life when I've been able to control it for certain lengths of time, and times I haven't been able to control it at all. (And of course during any time of my life how much either of those things are true varies a good deal.)
Right now I'm in a weird position where it's not really voluntary, and yet the movement isn't happening. I've explained this a few places. Basically most people don't notice this, but when a movement is trying to happen, there are several stages it goes through. And one of those stages feels almost like the movement itself, except that, if it's stuck at that spot in the chain of movements, then your body won't be moving. And these days due to my motor impairments that's where it's getting stuck.
So I have all these movements going through my head as if they are happening in my body, except most of them never get that far. So it's just like having nonvoluntary (not initiated on purpose, but suppressible to some extent or another) movements, except that my body doesn't give any feedback as if they happen. (And yet, it really feels as if they ought to. It's very strong, much different from simply imagining movement or something like that, far more immediate and visceral and very much like actual moving without being it. And then there's the odd few that do get through.
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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