That's a old model of supercharger.
What's "Blow Off
Valve" and what does it do beside giving the "pisss"/"whoosh"
sound?
To eliminate compressor surge when changing gears. Excessive
backpressure is created when the throttle is closed during gear
changes or deceleration, causing turbo cavitation. This is
detrimental to the life of your turbocharger; however fitting a BOV
will substantially improve this problem.
By installing a BOV you will significantly reduce turbo spool up
time. You will notice a substantial improvement in response from
your turbo between gearshifts. Boost will come on earlier giving an
increase in torque at lower RPM
The vast majority of turbo owners fit aftermarket blow off valves
simply for the signature noise that they make between gearshifts.
While that may be reason enough to fit one, they do actually serve
a more important purpose than that. A common misconception is that
if a turbocharged car doesn't make a "whoosh" noise, it doesn't
have a blow-off valve. Almost every modern turbo car will have some
form of blow-off valve, except that the vented air is returned to
the inlet tract before the turbo to muffle the sound.
A good analogy for what happens in a turbo system when you close
the throttle is water hammer in household pipes when you turn the
tap off too quickly. The same thing happens when your turbo is
spinning at about 100,000 RPM, pumping pressurized air into the
engine. When the throttle butterfly snaps shut for a gearshift,
that air (which can be traveling at up to 90 meters per second) has
nowhere to go except back the way it came and out through the
turbo. This is bad for a number of reasons. First, slowing the
turbo from maximum RPM to practically zero in less than a second is
obviously going to place a lot of strain on the turbo. Secondly,
since the turbo has stalled, the lag when you open the throttle
again is increased.
The BOV is designed to release excess pressure under these
conditions to allow the turbo to freewheel, which both prevents
damage and reduces lag. The noise is achieved simply by venting the
air externally.
You take a look here on some varies types of BOVs:
http://nextcollection.com/html/Auto_Performance2.cfm?category=Blow%20Off%20Valve&Department=Auto%20Performance
Hope this helps 
