When you try to squeeze out a fart with great force but you end up shating yourself, usually catastrophically.
EX:
Dude, I had gas last night after dinner with Ashley and I ended up poo poo farting on my way home in the car, The dry cleaners had a hard time cleaning that shit.
A fart that does not sound normal because it is blocked by poo. As a result this fart will also smell much worse because it has to travel past the poo to escape, picking upthe stench on the way out.