The built-in cassette body in the rear hub makes it possible to roles while riding without pedaling. In most cassette bodies are tiny pawls that can be folded counter to the direction of rotation. If you pedal, the pawls are caught in the hub casing. If no pedaling is carried out, the pawls grind, because of the oblique engagement over the recesses in the cassette body and the adhesion is interrupted. The typical noise of the free run is created. Without a cassette body on the bike, you would have to pedal permanently while driving, since the connection of the rear wheel with the crank through the chain results in a fixed gear ratio. This means that the pedals will continuously turn and a recovery phase in which you can "lift your feet up" would be made more difficult.