Uni-Directional Rotating Bezel —
a bezel that can be rotated in one direction only and is used to monitor elapsed time. A ratchet mechanism is often in place to prevent it rotating in the other direction. It is often found on divers watches to prevent the diver from running out of air by overestimating remaining air supply if the bezel is accidentally moved from the original position. The fact that the bezel moves in one direction only is a fail-safe feature which means the diver can only underestimate remaining air supply.