I’ve never really asked anyone about this, so I don’t know if this is perfectly normal, though I suspect it probably happens to a lot of people. See, if I’m sitting down, and put my weight on the ball of my foot, and sorta half-way tense up my ankle, my leg starts vibrating. My knee moves up and down a half inch or so, and faster than I could consciously manage. If I let it start, it’ll happen autonomously.
I particularly remember this from childhood, sitting in a desk at school. My father has exactly the same experience, and he also did it particularly often in childhood. Do everyone’s legs do this? Or only some people’s? Or are we just freaks?
Incidentally, I have mild restless legs, and so does my father. I wonder whether those things are related.