backfire-poop

Refers to where you are unknowingly expelling a "separated pair" of stumpy poop-logs (rather than just a "standard" one-piece turd of considerable length) with a sizable "gap" of pressurized methane in between them; the result is an explosive whooshy bang partway though the dump that echoes hollowly in the toilet-bowl and is amplified/intensified by said bowl, just like the sounding-board of a drum or stringed instrument.

A backfire-poop can be highly embarrassing for the pooper if others are within earshot at the time. Some guys, however, are actually proud of making these "impressive" anal indiscretions, since it proves that they only produce "manly-sized" turds which "seal in the combustion", with no blow-by from a minuscule-diameter poop-log's allowing the gas to wastefully/splutteringly seep past the emerging turd in a much-less-spectacular noise-display.