This is where you press with one or two fingers between the asshole and the ball sack to clear out the urine stuck in the "gooch trap".
AJ: Dude, I get piss that drips out after I shake and dance.
Kory: You've gotta purge the gooch.
AJ: That's right! No matter how much you shake and dance the last few drops will hit your pants unless you purge the gooch!
Kory: Exactly.
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 eatChinese 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.