Great porn sluts should comply to anything the director wants from them to complete his vision. Recently, the whip cream/white shit fetish appeared. The point is to fill the bitch's asshole with cream, then the guy can either have her to fart directly after the introduction or, better, give her a rough assfuck as the cream is still in her ass to built a lot of pressure in that hole. Usually, it also add a great look of discomfort on the girl face, a "did I make the best choice of my life being a fuck-for-money-hoe" which is delightful to watch. When the stud feels ready, he just have to remove his dick from her butthole and expose it like he would in order to show a nice gape, except that it's gonna be a loud and nasty creamy fart.
Everything I look for in porn is there : it's visual, nasty and extremely humiliating for the girl.