To expand on what Steve said...
The standard Uno Turbo has both a wastegate and a dump valve. You don't normally hear either of these because they are 'internal' and 'recirculating' respectively.
The wastegate opens to give the exhaust gas an easy path to the muffler, not going through the turbocharger. This prevents the turbocharger from working too hard, so the wastegate limits the maximum boost pressure (normal limit: 0.6bar). This is to ensure reliability of the engine by restricting the maximum temperatures and loads.
If you have a bleed valve fitted, it changes the point at which the waste gate opens. It's called a bleed valve because it bleeds off some of the pressure in the pipe to the wastegate opener (correctly called 'actuator'), so that the wastegate doesn't open until the boost pressure gets higher than normal.
I'm guessing that you have an 'external' wastegate, which I'm also guessing will release the exhaust gas directly to the surroundings? Apart from the noise I can't see any other difference in performance apart from probably having the actuator calibrated to a higher pressure than standard.
If there is boost pressure going into the engine, and then the accelerator is released sharply, the throttle plate closes and the boost pressure momentarily has nowhere to go - excess boost could surge back into the turbocharger's outlet causing 'compressor surge'. The dump valve opens in these conditions to let the excess boost go around into the inlet of the turbocharger (recirculating type), or an aftermarket external dump valve lets the excess boost out into the surroundings as a Tsshhhhhhh.
(This loss of boost then means that when the throttle re-opens, the turbocharger cannot build up boost pressure again until more air has been drawn in, which in so doing comes through the fuel injection system's airflow meter... causing the fuel injection to inject more fuel to match the extra air... and for this reason I personally don't see the point of an external dump valve, even though they are cool to have.)
Incidentally Biz is absolutely right in pointing out the very real purpose of both these components, which is why the Uno Turbo has both (wastegate+dump valve) as standard.
Hope this helps, please say if still confused
I'm thinking you must have a larger turbo as well to get 170bhp, so maybe you'd like to share what your setup includes?
-Alex