Freewheel

A freewheel is a device that can turn in one direction but not the other. It's kind of like a one-way road - you can usually drive forward, but you can't usually drive backward. When used on a bicycle, the pedals spin freely free when the bicycle is moving forward, but they don't move when the bicycle is stopped or going backward. This helps the cyclist to keep their feet on the pedals when they're stopped at a red light or going downhill.