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One player (the slappee) places his hands palm down, hovering above the other player's (slapper) hands. The other player hovers his hands below the first, palms up. The two players' hands should be barely touching each other, and all the hands should be around mid-torso height.
The slapper is on offence, and attempts to bring his hands over to slap the backsides of his opponent's hands. This must be done with sufficient speed, because the slappee's goal is to pull his hand away, and out of the area where the hands overlap, to avoid the slap. If the slapper hits the hands of the slapee during the slap, then the roles switch. However, the slappee cannot flinch too much. If slapper acts as if slapping (but does not ever bring his hand over) and the slappee flinches and moves his hands far enough from the slapper's hands so that their hands are no longer overlapping, then the slapper gets a free hit at one of the slappee's hands. If the slapper slaps the slappee's hands once his hands have left the overlapping area, the slappee gets a free hit at one of the slapper's hands, and the roles switch as if the slapper hits.