A simple way to tension a slackline is with a 3:1 system. This is also known as a Z-pulley system. It can be set up using pulleys, but for the purpose of a slackline carabiners can be used instead of pulleys.

The prusiks hold the tension in the system. I feel no need to remove the hardware from the system, as this is the same setup used in mountain rescue situations. If I were ever to set up a highline, I would back up this entire system.

The prusiks can be tended while tightening the system. Simply pull on the end of the rope, and then it is easy to hold the rope with one hand while you slide the prusiks back out to their extended positions with the other hand.

This prusik is only under tension during tightening; when you walk there is no tension on it.

To release this system, you need to pull the rope tight until the tension on the prusiks is released. Then hold the prusiks from biting the rope while you slowly let out the rope. This works as long as you can pull the rope slightly tighter than the last time you tensioned the system. For a more reliable setup, you can incorporate a release hitch into the system.