Rubber!
The leader has a rubber band at the base of the first two fingers of her left hand which is closed. She opens the hand suddenly and throws the rubber in such a way that it jumps clear across to the base of the third and fourth fingers of the same hand. Impossible? Not a bit, but only clever people can do it, or at least so the guests are told.
The rubber band is put at the base of the first two fingers. All four fingers are then folded over and the rubber is stretched a bit so that it may cover all four tips. When the fingers are stretched out straight it is inevitable that the rubber band jump to the base of the third and fourth fingers.