With a turbo, you want as little back pressure as possible so that the turbo can spool quickly, without resistance. That dump tube just opens up and becomes the exhaust pipe when he's racing so that he has zero back pressure.

When he's driving it on the street, he has it closed and retains the stock sound and look. That's sleeper baby! :yes:

:edit:

Oh, I see what you're asking. I think it works off of a knob style switch so you can adjust how much you want the dump open. I don't have a link to a site, but here's 3 vids (8MB total): http://josh.actworld.net/cutout.zip