same thing as a BOV. the only difference is where the excess air is put once the valve opens. the BOV will go to the atmosphere, and the bypass will put it back into the intake track PRIOR the turbo. some guys with stock turbos take of the piece that conects their bypass to their intake track to in essence make a BOV.
kinda hard to do on a bike though, since the intake track is pretty much an air filter....
alright this is a cheap good blowoff valve.look on ebay for a 1g dsm blowoff valve.they came on years 90 to 94 mitsubishis eagle talons ect...they will hold up to 22 lbs boost and they are all metal.they can be had with the flange for around $30 shipped they are pretty quite.they aren't the best looking things but they do the job they were built for fairly well
The spring pressures in a BOV have nothing to do with the amount of boost you plan on running, the spring pressure relates to the pressure differential at which the valve opens, You can run a BOV with a 7 pound spring to 100psi if you want. The different rate springs are for sizing for vaccum sources. In other words a v8 with a single throttle body draws a lot more vacuum then a vacuum nipple from a single throttle body on a 4 cylinder bike.