Tying a figure 8 mid-rope allows you to securely attach to a particular spot in the line. You can make hand or footholds this way, that can be easily untied after bearing weight. Learn to tie a figure 8 by watching our slide show series on knot tying.
The best knot out there for tightening and releasing cords that will be heavily loaded. It's perfect for cinching down gear, tensioning horizontal spans of rope, or getting a large tarp tightly pitched.