When your intestines are cramping up and make that 'girgling' sound a few minutes before you have to hit the can. Similar to a coffee maker percolating, your intestines are fixing up a nasty brew.
Feelingok, John?
Nah man, I'm percolating over here. Cover me for a few minutes, will ya.
opening your anus/butthole using your sphincter to allow air to enter. you then proceed to gain more air the same way. let the air sit then push out to perform an enormous fart