This is when a girl is getting
fucked up the ass so hard that both her and the person
fucking her knees buckle from the forces of fucking and
getting fucked. Best observed in any porno where anal poundings are involved.
Dude me and my girl were fucking, it was
Knee Buckling Butt Fucking night, she was so tired after that she fell asleep with my cock in her ass, what a night!
Knee Buckling Ass Fucking is the way to go