A retractable fall arrestor block is used in lifeline systems to provide continuous fall protection while allowing freedom of movement. It works like a seatbelt: the line extends and retracts automatically as the user moves, maintaining minimal slack.

This reduces fall distance and limits impact forces if a fall occurs. In the event of a sudden drop, an internal braking mechanism engages instantly, stopping the fall. 

The design improves safety, efficiency, and ease of use compared to fixed lanyards, especially in environments where workers need to move vertically or horizontally while remaining securely attached at all times.