That definatly looks like a blow off valve. Prevents compressor surge, thats about it. Oh and it makes a nice pssht sound when you let off at WOT. :biggrin:
LOOKS GREAT and has functionality..... I wont go that route... cant have alky releasing all over the engine compartment... for some reason it just doesnt seem safe
A nice setup for some BOV's is the 'dual vent'. What this does is allow air to be recirced back into the intake 100% like a recirc valve under low to moderate pressure. When the pressure is strong enough to open the valve fully it sends the first ammount of air back into the intake and the excess air is vented. It is a pretty neat set up and many of the different versions allow for adjustment of how much air is recirc'ed and vented. Alot of guys with WRX's and STi's prefer the dual vent style valve.
Here is the APS dual vent: http://www.airpowersystems.com.au/wrx/blow.htm