When someone with a fartfetish (one who is erotically & sexually fixated with farts/farting) fantasizes and/or masturbates to their object of fixation farting, they do not focus on the stench of the fart but of the noise made by the personpassing gas, both from the anus and the erotic moaning they may add to simulate extreme pleasure during the act. This term can also stand for the actual act of one farting into the face of another - such as during face sitting, ass worship or stink facing; the person "receiving" gas is, more than likely, focusing on the previously mentioned factors and not the smell itself. Hence the name - "fantasy fart": there's no way you can't smell somebody passing gas - it is, more or less, a "fantasy".
1) Adam masturbated as he fantasized about Carla mounting his face, blowing a fantasy fart upon him.
2) Marisa squatted above Julio's head, fantasy farting as she lowered her booty to face sit upon him.