We've all seen it somewhere. A character (usually male) shakes the hand of another (usually male) character in supposed friendship only to try and crush the character's fingers. Usually done when the crusher either a) intensely dislikes, b) wishes to intimidate, c) is competing over a girl with or d) is just that mean to the chrush-reciever. Can be subverted in when the crusher's fingers get hurt instead, either to do with the other character being stronger than they look or made of steel.
Hello, Unknown Troper. You'll need to get known to lend a hand here.