You typically yell this in a state of panic as you can feel the shit trying ever so desperately to jump out of your butthole. You clench like your life depends on it. Which in this case it lowkey does.
Oderous flatulence which proceeds a large shit (pre-poop). Poop farts can yield information related to the upcoming crap including degree of stench, mass and potential anal velocity. Those conversant in art of poop farting can potentially predict color, texture and volume of the ensuing shit.
I had too much Qdoba last night and based on these poop farts, I predict a double-flush power dump with green, furry logs.
(While taking shits in the locker room)
Rick: "Hey Sid, what was that menacing sound?"
Sid: "Oh it was just my Pre-Poop Fart, I ate WoW chips before practice."