Get into a squat position as if you were a catcher in a baseball or softball game.
Extend one of your legs. It really doesn't matter which, most people simply use the one that feels more comfortable. This extended leg is the "blade" of the helicopter.
Place your arm of the leg that extends on the ground, knuckles down. This is going to be your pivot point as you rotate the "helicopter."
Wrap your extended leg around until it just about touches your crouched leg.
Hop off your crouched leg as you simultaneously use the hand that is on the ground as a pivot. While your crouched leg is in the air, your extended leg will sweep underneath.
Repeat this move as many times in a row as desired to create the effect of the helicopter blade whirring around and around.