I tend not to notice I've started, but sometimes I get a compulsion to stim which I'm aware of.
I usually only become aware of my stress-related ones and let myself continue because they help with it! But if I'm feeling insecure or embarrassed I can switch to a less noticeable stim. One of my main stress ones is curling my fingers and wrists back, then flicking and twisting them side to side. I tend to stop if someone points it out or I get weird looks and switch to tapping my thumb with my fingertips in a repetitive movement.
I'm less aware of ones I do when concentrating or happy unless someone points it out to me. Hand flapping, jerky movements where I can't keep still, noises etc. I'm usually too excited to stop or find it awkward to maintain my concentration without doing it, but there are ones people don't usually comment on, such as chewing plastic or pen lids, slight rocking, tapping etc.
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ISTJ / ASQ = 37/50
AQ = 143/200 NT = 62/200
?Perhaps one did not want to be loved so much as to be understood.? George Orwell, 1984