The Munter hitch, probably known more correctly as the Italian hitch, is a useful knot to know for climbers and abseilers. It can be used as a descender, and as a belay device for climbing, if you don't have an alternative.

If you haven't used it before, make sure that you test it under controlled conditions before using it in a real application.

Munter hitch
Form a loop in the rope as shown Munter hitch - slide 1
Flip the loop over itself Munter hitch - slide 2
Clip a carabiner around the double section and through the loop Munter hitch - slide 3
This is how the Munter hitch will look when used as a descent device. The rope running from the top of the carabiner is taking the strain, and the rope coming out the bottom goes to the brake hand. Munter hitch - slide 4