Sucking a fuck is a combination of a blow job and straight up fucking. The chick is on her back, while the dude literally fucks her in the mouth forcefully thrusting his dick in and out of the hole on her face. A bit of flexibility is involved for the guy ...and a whole lot of props for the girl.
Girl, last night I was giving this dude a blow job and he climbed on top, mounted my face like a spider monkey, and humped my face. I now know how to suck a fuck.