The sphincter, a.k.a. butt hole, when gas or shit or both are blowing out of the ass either due to the need for immediate pressure relief or a necessary bodily function. Use of the Purge Valve may often satisfy both.
Gary went to the all you can eat Chinese Buffet where he consumed a massive amount of beef and broccoli for dinner. A few hours later he was so bloated air began forcefully blowing out of his Purge Valve. He soon felt relief and his pants weren't so tight.
by A. M. Wood June 19, 2009