why do athletes slap their butts to congratulate or to cheer them on?
Some theories behind the butt smacking links it to the horseracing. When the horses are struck on their butts, they encourage them to go faster. In so doing, the athletes have also picked up the habit to congratulate and boost each other's morale.
To loosen muscle, and prepare themselves to get ready.
it's to get them pumped up and to encourage them. its also used to tell them good job.