you hold your fists so the knuckles touch (one hand from each player), then one person has to try and hit the other's knuckles with his OR feint at them. If the hitter feints and the other guy flinches (moves his fist away) the hitter gets a free hit. If the hitter makes an attempt to hit the knuckles and misses, it's the other guy's turn to be the hitter.
you can also play it as "slapsies" where you have flat palms, together in the praying position, touching at the fingertips, and attempt to slap the back of the other fellows hand with one of yours