Carski objanseto...
http://www.max-boost.co.uk/max-boost/intake/DV.htmHere's how it goes: Happy Larry tries to impress his friends, and gives it some stick. Full throttle creates a rush of air in the cylinders, lots of exhaust gases are produced, the turbine spins like hell, the compressor is also forced to spin at the same speed (they share a common shaft!) so lots of boost (compressed air) is being shifted down the pipes, through the wide-open throttle. Larry's happy.
All good things come to end unfortunately, either the long straight ahead is over, or blue lights are flashing in the rear-view mirror.
Time to get off the throttle - suddenly the throttle plate is SHUT, but all that boost has nowhere to go. It's trapped between the throttle plate and the compressor, that still spins for a fraction of a second, as inertia and a few remaining exhaust gases still try to spin the turbine. That is a wall of pressurised air that won't let the compressor turn as fast as it needs to - but the turbine doesn't know that yet (it will, in a while, because no combustion creates no more exhaust gases, but it takes some time to respond).
So the turbine tries to spin the shaft, but the trapped boost forces the compressor to stop spinning. That can't be good for long-term reliability. Run a bit too high a boost and after a few gearchanges something will .
This condition is compressor surge, and it means that the compressor cgivereates boost that the engine can't swallow quickly enough. A very distressing sound usually accompanies surge. It can easily turn ugly, with shattered wheels or shaft bits all over the place.
The dump valve prevents this condition, because as soon as the throttle is shut, it vents the excess pressure from the pipes, preventing this surge. Women love it, too. 
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Sto so fabrickiot recirc.valve?
When fitting a dump valve you can't just leave the stock recirc valve lying there without the vacuum hose attached. This will lead to boost leaks. The recirc valve has a weak spring keeping it closed, it can be pushed open by just a few psi worth of pressure. This spring is meant to work in conjunction with the vacuum (and boost) pipe: Whenever there is boost on the inside of the valve, there is meant to be equal boost on the other side of the valve too (via the pipe). These pressures will cancel each other out, and the weak spring will be enough to keep the valve closed.
A good idea is to install a DV and also keep the recirc valve in place as well. Both can be hooked to the vacuum pipe with T-junctions. There's enough vacuum to keep everybody happy, including the Fuel Pressure Regulator that's also using the same vacuum source.
The advantage of this setup is that there is a lot of recirculation, which is a GOOD thing, as less unmetered air escapes. Also the dump valve doesn't work as hard, so it will last longer, and it will be quieter, making the car more 'stealthy'. Responsiveness is also increased marginally, as the recirc valve is as close to the turbo compressor as it gets. The overall dump volume capacity is increased as well, with both working together.
