When the bottom of a persons ass cheeks are not covered by their bikini because the ass crack gobbles them up or because of the ass is full and can't be contained.
A atrocious fart that comes out your buttox between your right and left booty cheeks. To prevent sound you lean forward and shove your buttox backward, then sit as fast as you can, flat, to prevent a really ear-rippling sound that your comrades will hear.