[sperry]

Like Austin said, there's basically two "fans" inside a turbo. One fan is in the exhaust stream, so the flow of exhaust makes it spin at a rediculously high speed. The other side of the turbo had a fan that compresses the intake air.

So when te exhaust flow is fast there's more compressed air entering the motor. And more air = more oxygen, and it's the exploding of the oxygen that makes power.

Like Mike was saying, because there are two different fans in the turbo, there's an intricate balance to be made between the sizes of the fans in relation to the motor. You can achieve different results by using different sized compresser and impeller wheels. I'm not privy to the details (it's over my head as to exactly how it all works out) but in general a smaller turbo needs less exhaust to spin up, so a smaller turbo has less lag, however it can't flow a lot of air so it limits top end horsepower. While a larger turbo can flow a lot more air so it's goot for big horsepower numbers, but it takes a long time to spin up so there's a lot of lag.