What is the function of an auto block in a rappelling system?

An auto block functions as a safety device within a rappelling system, primarily designed to prevent out-of-control descents. It works by acting as a friction device that automatically engages when excessive force is applied, such as during a rapid descent. This mechanism allows the user to maintain control over their descent speed while providing an added layer of safety in case of unexpected slips or loss of control.

By positioning an auto block on the rope above the rappel device, it can effectively lock in place if the rappel device is subjected to a sudden pull or rapid movement, thus preventing the user from falling further. This feature is crucial, especially when operating under challenging conditions or in high-stress environments where the risk of falling exists.

The other options define different components or methods in rope rescue but do not capture the specific role of an auto block in enhancing safety during rappelling.