Zip lines are great fun but how do you slow them down? You need a zip line brake. A Bungee brake is a safe way to slow every zip line riders down without worry. Mount the block near the end, attach bungee cord to the block and anchor the other end of the bungee cord to an adjacent tree. The rider will zip down the cable and the trolley will bump into the block, which engages the bungee and slows the rider down. Use a 20 FT Bungee for zip lines less than 200 FT. Use a 30 FT Bungee for zip lines less than 500 FT. Never use a zip line slope greater than 6%.