Most commonly used by adding bungee cord or a rope (neither included) to slow you down at the end of your zip line run. This block works great with 20' or 30' of bungee and zip lines 100' or longer. This Trolley Brake Block is a perfect companion to nearly any zip line. It is used to help slow down the rider at the end of the zip line. It is made out of a high quality, dense recycled plastic designed for exceptional wear resistance and long life. (Comes with all attaching hardware and backup cable sling).