I believe it has to do with practice. For example, when I was a teenager I became obsessed for a few days with bending only the tips of my fingers, at the first joint. I would actually practice by holding the fingers still with the other hand and bending only the tips. I can do it with my right hand because I practiced with it(Lefty). I also taught myself to cross one eye and keep the other straight. It was simply crossing the eyes and then looking left or right. One eye stays crossed if you do that. It was then finding the midpoint so that one eye was halfway in between leaving it straight. I can do that with both eyes. I can also bounce my pecs independently.
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