It's when a male mounts an individual's back while the person is on their stomach. The male is reverse cowgirl and then proceeds to fuck the ass crack of the individual as if he were tittie fucking the shit out of the individual. To reach the fullest potential, the male then cums and causes the cum to run into the butthole, making it complete.
"Hey Mike, remember when Bartonian drank that whole bottle of vodka?"
"Yea, man, Harrito gave him one hell of a Pittsburgh Piggie-in-the-Blanket!"