On a turbo charged car, there is some evil dude called turbo lag. Turbo lag means that the car lags in the beginning of the rpm range due to the fact that turbo does not kick in from the very beginning. However, when it does, you'll ofcourse feel the thrust. Therefore, some cars with twin turbo uses 2 tubo systems, one for low rpm range, and the other for high rpm ranges. The size of the 2 turbos are ofcourse different. THat way, the twin turbo greatly reduces turbo lag because there is a smaller turbo to push the car without turbo lag in the low rpm range and the 2nd turbo (which is bigger) takes over. ViolA!