here is a little explaination to make it a little clearer
The Whipple Charger
The screw compressor is by far the best supercharger available today; it’s the only supercharger that is able to provide a positive displacement/internal compression design for maximum low-end torque as well as high efficiency for maximum top-end horsepower. No other supercharger in the world can offer the same unique benefits. Centrifugals and turbos have tremendous lag time during shift points, on-off throttle and off the line boost. Roots-type superchargers have been proven to not be as efficient as the screw compressor due to the external compression design, especially at higher boost levels, which greatly affect peak horsepower. The screw compressor’s unique design and efficiency allows for compact installation and OEM reliability.