You find a girl and take her down the beach. You fuck her and then you dump your man paste inside of her, kick sand in her face, and run like hell, never to be seen or heard from ever again.
The area between your a hole and your sack where things occasionally get stuck, hence the sand trap. Items often found lurking in the sand trap include lint, stray hairs, and occasionally dingle berries. Sand trap also referred to as the grundle.