Have you wondered what is that tick-tick-tick you hear when a bicycle is coasting?
Photo by Keithonearth
A pawl is a small flipper device that engages a ratcheted surface so rotary movement is possible in one direction only. Freewheels typically have two pawls. Most pawls are spring-loaded, but they can also be centrifugally actuated. The images above and below are Sturmey-Archer 3-speed hubs containing pawls. The picture above is spring-loaded. The pictures below are from an older style hub that are centrifugally loaded.
Below are pictures of a Shimano freehub, also known as cassette hub. These are very common freewheeling systems found on most derailleur equipped bicycles. These contain a ratchet pawls within the hub.