zipBACK: Zip Line Retraction Device Increase throughput and reliability on your mini zip with the zipBACK Retraction Device. The zipBACK is a simple, reliable, and costeffective solution for mini zip line rider trolley retrieval. Compatible with zip lines up to 40m (130 feet) long with a simple external 2:1 redirection system supplied and installed by the end user. To be used only in nonlife safety installations to retrieve rider trolleys.