'How To Tie a Slip Knot' is part of the book - Trad Climbing Basics.
The slip knot is useful for tying off pitons, tree stumps or other poor gear in order to reduce leverage.
How To Tie a Slip Knot
Pull a bight through this loop as shown.
Slip this bight over the piton.
Cinch it tight and push it as close to the rock as possible to reduce leverage.