When you and your partner are having sex, one partner gets into doggy style receiving position while the other partner gets on top of the other partner as a person might mount a horse. They then continue to defecate upon their partner's back and slide the feces around all over the partner's back. The slop upon their back and the other partner's genital area resembles a saddle.
Katie: I was having sex with Joe last night, and went into doggy position when suddenly he got on top of me and pulled a dirty saddleback!
Sally: Oh my gawd! I wish Stan would do that. So hot!!