When you break down your slackline, you need to release the tension in the system. A simple and reliable way to do this is to use a release hitch. I use a radium release hitch, because it is the one used in mountain rescue work. This hitch is a 3:1 system combined with a munter's hitch.

Instructions for tying a radium release hitch can be found on this page.

To release the tension in the system, unwrap the extra cord in the release hitch and feed it out slowly through the munter hitch. The actual release hitch is quite simple; most of this rope is the extra length necessary to feed through as tension is released.