Close to the bump; the side splash is an affective hand gesture when one party is excited for a decided outcome that he offers the normal fist out to the side to be pounded for recognition.
Though after the two fists connect them abruptly splash with all the fingers wiggling in a 'water' like motion and followed by the noise 'psssccchhhttt'.
It is normally done by tough guys who play casino side bets such as 'Lucky Lucky', 'Pairs Plus' or "Super Sevens" when the bet 'pays off'.
This is mostly done with a nonchalant type expression on both parties face and is one of the most possibly gay things to do when playing any casino game.
Tough Guy 1: "Oh shit, suited 21!"
Tough Guy 2: "Side splash!"
Tough Guy 1+2: "Psssccchhhttt!"