This sexual move occurs while having sexual intercourse with your partner from behind (doggy style) in a bathroom in front of a toilet. Right before your partner orgasm's, whether male or female, you shove their head into the bowl of the toilet.
Guy1: You know how i got with that chick last night, the busty brunette.
Guy2: Yeah what about her?
Guy1: I totally Dunk Tanked her last night!
Guy2: YOU DIRTY DOG, YOU!!!