Belay devices are mechanical pieces of climbing equipment used to control a rope during belaying. They are designed to improve belay safety for the climber by allowing the belayer to manage their duties with minimal physical effort. With the right belay device, a small, weak climber can easily arrest the fall of a much heavier partner. Belay devices purposely do not instantly 'lock' to arrest a fall, since an instant stop from a fall during lead climbing would probably injure the climber or cause all the protection devices to fail. Instead, belay devices act as a friction brake, so that when a climber falls with any slack in the rope, the fall is brought to a stop over a finite (but reasonably short) distance.
Belay devices generally have two modes of operation: In the first mode, the rope is relatively free to be payed in/out by the belayer. The second mode, which allows the belayer to arrest a climbers fall, functions by forcing the rope(s) into tight bends where the rope rubs against the belay device and/or against itself. This rubbing slows the rope, but also generates heat. Some types of belay devices can transition between these modes without the belayer taking any action, others require the belayer to hold or pull the rope in a particular direction to arrest a fall.
Belay devices usually attach to the harness of the belayer via a carabiner, and are usually made of aluminium or an alloy. Some belay devices can also be used as descenders for a controlled descent on a rope, that is abseiling or rappeling.