In order to decide what color you will use when playing chess, first hide one unit of each color in each of your fists, then stretch your fists out towards your partner and let him/her choose one. When he points at one, unfold your hand and reveal to him the color of the unit and that will be his color. Your partner can just point at it or tap your fist, or even just nod at the fist he chooses.That was the slap he was talking about.