A rachet involves a round gear or a linear rack with teeth, and a pivoting,
spring-loaded finger called a pawl that engages the teeth. The teeth are uniform
but asymmetrical, with each tooth having a moderate slope on one edge and a much
steeper slope on the other edge.
When the teeth are moving in the unrestricted (i. e. forward) direction, the pawl easily slides up and over the gently sloped edges of the teeth, with a spring forcing it (often with an audible 'click') into the depression between the teeth as it passes the tip of each tooth. 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 protecting against any further motion in that direction.
Because the ratchet can only stop backward motion at discrete points (i. e., at tooth boundaries), a ratchet does allow a limited amount of backward motion. 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. The pawl bears against the surface at an angle to ensure any backward motion will cause the pawl to jam against the surface thereby prevent any further backward motion. Since the backward travel distance is generally a function of the compressibility of the high friction surface, this mechanism can cause drastically reduced backlash.