What do dump valves REALLY do?

[nedge2k]

Do they actually protect your turbo or are they actually causing lag?

I've always been of the opinion, rightly or wrongly, that a dump valve is used to protect a turbo from stalling or spinning back once the throttle plate is closed by releasing the built up pressure and allowing the turbo to spin freely. The side effect of this being that the turbo can then spool up quicker and lag is reduced.

HOWEVER

Whilst reading up on a few things, I came across an article from an Australian tuning firm that rekon the only thing a dump valve is good for is CAUSING lag. According to them, the only reason manufacturers fit them is to control emissions. They used the example of a rally car - I can't ever recall hearing one with a dump valve but you do hear "flutter dump" / "surge" / "pigeon" whatever you want to call it. Now obviously, a rally turbo only has to last a few hundred miles on each event so it's probably not a great example.

I do prefer the sound of "flutter dump" to that of a dump valve....but not at the expense of my turbo. I don't see how not having some kind of relief valve, even at low boost, could be a good thing? I mean, that pressurised air has to go somewhere, the only place is can go is back through the turbo and I just can't see how that couldn't do damage? That article doesn't exactly do much to allay the common fear of turbo stall and damage so, does anyone know what the real deal is?