Pop-off valves in STX?

[Jake Hodges <jake@codeworm.com>]

I'm asking for a friend..  I don't actually know what a pop-off valve is, 
but apparently it's desirable in some way.

1. Is it legal in STX?

2. For my own information, what does it REALLY do?  I've heard some "fast 
and furious" explanations, if you know what I mean, but I'd like someone 
with some understanding of the English language to show off how much they 
know on the subject to help my friend make an informed decision.

Thanks, and I love you all!

Jake