How does a belay device work?

• The opening guarantees a minimum of wrap, or bend, in the rope to produce enough friction on the post.
• The post is usually a locking carabiner or part of the device itself.
• The opening must be large enough to allow the device to touch the carabiner.

How does the belayer stop the fall?

To stop a fall, the belayer pulls back on the free end of the rope to create a separation of at least 90 degrees between the rope entering the device and the rope leaving it.

• Nothing must be in the way of your braking hand or elbow carrying out this critical task, and it must not require an unnatural body twist or motion.
• The simplest way to learn to do this conveniently in all situations is to clip the device into a locking carabiner on your harness (currently the most popular belay method in the United States) rather than directly to the anchor.

Contrary to a popular misconception, there is no automatic clamping effect with most properly designed belay devices. Your hand is the initial, and critical, source of friction; without your braking hand on the rope, there is no belay. The total friction is determined by the strength of your grip, the total number of degrees in the bends or turns the rope makes, and the rope's internal resistance to bending and deforming against the sides of the device and carabiner.

The fact that most belay devices depend on the belayer pulling back on the rope to stop a fall can be a disadvantage, especially when belaying a follower. The belayer often is unable to see the follower-and tests have suggested that the normal reflex, when an unexpected fall occurs with the belayer unable to see the falling weight, is to grip the rope but not to pull back. Thus the two strands of the rope remain parallel, and the belay device produces very little friction, allowing the rope to run through. The result in tests is usually that the weight is dropped to the ground. In a real situation, the result would be a longer fall but not necessarily total failure of the belay. It's possible that through practice, using a weight-drop setup, a belayer would develop the proper reflex.