How do you stop on a zip line?  You can set the cable so gravity slows the rider, then they bump into the Stop Block at the very end. You can add a Spring Brake to provide a little cushion upon impact with the Stop Block.  Springs are designed for short zip lines with gentle slopes around 3%.  You can also use a Bungee Brake which is designed for longer rides greater than 100 FT, with up to a 6% slope. For a fast ride and gentle stop, grab a bungee brake.  EVERY ZIP LINE MUST HAVE A SAFETY STOP BLOCK. Match the size block with the gauge of cable you are using.