rumble shits

Rumble shits is a natural phenomena that can not be forced/nor predicted. Seconds before you're about to drop a deuce, a series of loud, echoing flatulents exit the ass. Scientists believe that rumble shits exist merely to prep the butthole for the events that follow.

When you get belly rumbles/ pains which usually occur before you need a poo.
When your poop is in process of exiting the anus.