As Red bull says, it has a small bore through the centre, enough to flow for low power, then when you hit the GO pedal, centre part is pushed off its seat allowing a much larger diameter.

It's hard to say whether my car felt slower with it in, it was SO much quieter that of course it seemed less brutal. We did dyno a 400bhp 200sx with one of these, 2 back-to-back runs, not even 1bhp in it.