how do blow off valves work, need one that can work on a briggs 10 hp

[chris1zz]

supercharging a b&g 10hp ohv engine, but I need a convient way of regulating the boost air pressure. Is a blow off valve going to do this for me? Are blow off valves self regulating? How do they adjust? I was looking on ebay and it looked like all the valves have a vaccum line nipple?? Im guessing that when the throttle is lifted and the manifold pressure goes neg, the bov opens? Or, does the bov sense the air pressure and open up when it reaches x psi?

I need the later.

Thanks guys!