The act of allowing the girl to climb on top of you. You then immediately slap the right side of her ass as if you were a cowboy slapping a buffalo. Once her eyes light up and she realizes she was just branded, she will give a stare just like a buffalo. Buffalo slapping becomes even cooler while partaking in Hate-Sex.
"Dude, she wanted to try something new, so I buffalo slapped her."
"Man she was hot last night, i kind of wanted to have hate-sex with her and buffalo slap that shit.