A ratchet consists of a round gear or a linear rack with teeth, and a pivoting, spring-loaded finger called a pawl (or click, in clocks and watches[1][2]) that engages the teeth.
When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.
This backward motion—which is limited to a maximum distance equal to the spacing between the teeth—is called backlash.
In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as rubber is sometimes used.
Since the backward travel distance is primarily a function of the compressibility of the high friction surface, this mechanism can result in significantly reduced backlash.