When theBig Bad wants to flex his power, he shoves his hands into big (usually black) gloves with anupward thrusting motion, then makes an evil fist (although a clawed semi-fist, as if clutching something unseen, is sometimes also done). Putting on the gloves can be optional; the character can make this move even if they're already wearing the gloves. For extra points, we get a scrunchy-leathery-rubbery sound effect as the fist is clenched.
Pretty much aMad Scientist staple, but the fisted glove is also a trope of megalomaniacs in general, showing off their power.
Could also be anEvil Hand. When it's aMad Doctor performing it, it might also include aGlove Snap. When the heroes do it, it's aBicep-Polishing Gesture. CompareMilking the Giant Cow. Not related to aFist Pump. Not to be confused withGiant Hands of Doom. Also not to be confused with thePower Fist. Or theBlack Power Fist.