It's almost entirely dictated by my level of physical energy. When I'm calm or tired, I don't stim much. When I'm excited, anxious, interested, thinking hard, frightened, or angry, I'll stim so much you'd think I couldn't do anything else; but really, I'm probably thinking very quickly and often quite efficiently. This is notable because both "good" and "bad" states can make me stim more. At the very top of the overload scale, near-meltdown, I don't stim much at all; what I do will be a single, repetitive thing, often banging my hand or head against something, rubbing or pinching skin, usually not hard enough to cause injury, just trying to overwrite the stuff that's getting to me. Could be that some of the stimminess is also ADHD fidgeting. I do some of my best studying while in constant motion.
I wonder how many other autistic, especially children, just get too overloaded to move much, and whether it is perceived as a good thing by the people who care for them because they're stimming less...
Could be one of the reasons I'm OK with predicting/preventing meltdowns: At the anxiety or frustration stage, before I lose the energy, something about the constant-motion stimming tends to give me enough mental energy to avert the problem before it gets out of hand. If I deal with it at that point, I never quite lose control and go to the sluggish, repetitive sort of thing that happens just before I lose it.
Many of my stims are muscle-twitches and aren't extremely visible; but the rocking, flapping, finger-tapping, leg-jiggling usually shows.