How to tie various knots

Step 10 Prussic Knot

The prussic knot may be used as a climbing ascender - when made around a vertical rope as shown, downwards tension on the ends locks the knot, while release of tension allows it to be slid up the rope. A pair of prussic knots on foot loops may be used in tandem to climb a rope.

Uses:
Climbing a rope
Taking up tension to free a riding turn on a winch