The rear sprocket is normally attached to a freewheel hub that permits the rear wheel to rotate faster than the speed dictated by the advance rate of the chain. In the photograph below, the freewheel ratchet mechanism is inside the set of cogs that permit the overall gear ratio to be changed. A "Derailleur" mechanism is required to move the chain between the gears. Some bikes may use a similar mechanism at the chainwheel to increase the number of gear ratios available.

Spring-loaded ratchet pawls can be seen inside the cog set. These engage in the grooves on the outside of the freewheel hub and permit relative motion of these components in one direction only. Sliding friction between the pawls and the hub in this assembly during freewheeling dissipates some energy, but the comfort of the cyclist is greatly increased. Sprint bikes used a fixed rear hub for better control.

From: Sloane, "Sloane's Complete Book of Bicycling," Simon & Schuster (1995)