a dump valve is basically a way of venting built up pressure to the atmosphere

so; when your turbo is making boost you have pressurised air, now once you come off the throttle; the pressurised air needs somewhere to go.

if you dont have a dump valve the pressurised air can only come out of the turbo (which is why you get that lovely fluttering noise) this is better known as turbo stall, because essentially the air is stalling the turbo. However if you have a dump valve the presurissed air will come out through the dump valve which is why you get the ptshhhh noise.

This is why with a DV fitted the turbo doesnt have to spin up again which will in turn make the car feel faster between gear changes

Hope that helps; i'm crap at explaining things!

Matt