PoP off Valve, Article in Perforance Boat
 

In the lastest issue of Performance Boat magazine, there is an extensive article about someone who made there own pop off valve.

Usually I deduce through the article what they are talking about, if not I am the first one to advocate searching the forums and the rest of the net.

However, in this case, I can not figure out what a pop off vavle does, or where its located.

Sorry for a remedial question, just wanted to learn a lttle bit more.

Any insight?

Thanks