Restrictions to an engine breathing and turning causes the engine to use its power to turn against itself instead of just the transmission and driveline. Allowing the engine to breathe (in and/or out) easier or reducing friction inside the engine (roller bearings) requires less energy from the engine to create the power that is needed to turn the wheels. Take off a pulley from the belts and there is about 10-15 extra HP (=increased efficiency) becuase of less resistance. Add roller rocker arms, roller cam shaft, roller timing chain (older engines) and you have increased efficiency. take off exhuast or intake restriciton or in the srt6 case, actively push it in with a supercharger.

An example is the new ford power strokes have two turbos to push the air through the increase regulations of more exhuast cleaning (increased restriction). Take away the restriction in the exhuast, and wala, you have increased fuel economy (also more power and throttle response). The two turbos were required to maintain the same power due to the new exhuast restrictions. It uses more fuel to push past the increased restriction......