When two people lay in the missionary position and create a pocket of air between their stomachs. When more pressure is applied, the air is release making a farting sound. This mostly occurs during sex or extreme make-out sessions.
Guy 1: "Dude, last night i could not stop laughing after all those chest farts while me and Amber were gettin' it on." Guy 2: "Hah, always gets ya. Damn sweaty chest farts."