I've just encountered the term "stim" on this board tonight, though I already knew the concept(s) it refers to.
In terms of the original question, I find them to be usually *voluntary*, because it feels good.
One thing I will do is go into a kind of voluntary catatonia. If I'm *really* stressed, I will find a spot, scream at the top of my lungs, curl up and just stop moving or responding to anyone. I know exactly what I'm doing, and I can stop at any time, but I just can't handle the stress anymore, so I jus tclose myself out.
Not sure if that's a stim or not.
I will do many of the things mentioned in this thread. Sometimes, I become absolutely fascinated by some image and just stare at it (in terms of chickens and eggs, i am also visually impaired, almost legally blind, so I like to "rest my eyes"). I might do something either involuntarily, accidentally, or ona whim, and think, "Hey! This s neat!" And just keep doing it, which will drive my wife nuts. It may be playing with something weird (yesterday, I confiscated a broken Easter Basket handle that my son was whacking his sisters with. It struck me that it resembled the mandible of a giant spider or ant in a movie, so I put it around my head and pretended i was a giant bug). Or I'll speak in Seussian rhyme, or a Southern accent, or an English accent, or sing everything I say, or wahtever, till she threatens bodily harm if I don't stop.
But, as others have said, it usually starts as something involuntary or accidental, and I think, "hey! This is neat!"