I know what it does and how to hook it up an stuff, but how exactly does it work? basically like a wastegate?

 

positive vacuum pressure pushes on the valve to keep it shut, when the pressure in the intake mani is cut off the pressure is greater in the chargepipe thus forcing open the valve.

 

That was kinda confusing.

I'll give it a try in english. :P

When you are out of boost idleing, you have around 20lbs. of VAC which actually opens the bov in some cases while at idle. When you hit boost the vac is gone and boost is in the charge pipes which keeps the valve shut. When you let off the gas the throttle body closes and boost pressure hits the valve along with VAC pressure which opens the valve releasing the built up pressure.
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So, the top nipple is VAC, from the manifold, and bottom is connected to the charge pipe right? okay, so, closed throttle theres that 20 pounds of VAC you were talking about, which "sucks" the plunger up, causing it to release boost into the atmosphere. now, when in boost, what keeps the 20 PSI that you are running in your charge pipe from pushing up the plunger up?

EDIT: NVM...Theres that 20 PSI from the manifold going into the nipple on top when open throttle that basically negates the 20 PSI in the charge pipes, right?

 

lol, you got it.
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20 mm/hg not psi, :P