Here it is, typed faster than the speed of light (or copied and pasted, lol!)
Right, at the very simplest level, engines create power by exploding petrol, in order to do this they need air.
The amount of air that an engine can consume is limited by many factors, including the cam, the breathing limit of the head, the size of the valves and the size of the engine itself.
One way to overcome all these limits in one go is to go for forced induction, this is the general term for any device that rams air into the engien at more than atmospheric pressure, ie turbos or superchargers.
The way a turbo works is that it basically has two big fans in it, which are mechanically attached to each other, so that when one spins, the other spins as well.
The exhaust gasses from the engine pass throguh one fan, making it spin, this in turn spins the other fan, the other fan is in the inlet path for the air coming into the engine, so it bascially blows a whole load of air at the engine under high pressure, thus allowing it to get more air in (and hence burn more fuel)
Now there is a problem here, when you back off the throttle to change gear you close the intake path more or less completely, this means that the turbo has nowhere to push the air to, so it makes it stall, this is bad for a couple of reasons, firstly it means it has to spin up again, which means a temporary loss of boost when you get back on the throttle and secondly it means that there is perssure on the turbo itself, cause one fan is trying to stop while the other still wants to keep spinning, this can shorten the life of the turbo.
Now what a dump valve does is detect the difference in pressure when you shut the throttle and basically open a valve (i.e makes a hole) that the pressure can escape through to stop it bouncing back at the turbo and stalling it, this vent of gas is the noise that you hear.
Most production cars (even the metro turbo!) have a dump valve fitted as standard, but they are silent operating (more or less if you really listen you can hear them sometimes).
On hugely powerful rally cars and stuff the turbos are running so mcuh boost that the standard valve cant cope, so they install these loud after market ones instead, on a lightly modified car though you will see no benefit from one, hence my opinion of them as tacky, its making a load of noise trying to make let everyone know you have a turbo, bit like having turbo written up your car or whatever, i personally prefer things to be kept a secret, people can find out when they race me instead!
Thats only a VERY brief explanation and there is a lot of detail missing, but hopefully its enough to help you understand the principle of dump valves, and even a little bit about turbos as well.
