In the first diagram stands the line which is jammed. You wish to take the pressure off it by pulling it to the right.
One round turn is made
And then another. The bitter end it passed under the standing part
Another round turn is made, and the bitter end is passed upwards as shown.
Strain is now applied in the direction of shown. This knot needs to be kept under strain, once you start winching on it, it will tighten right up and not slip.
Thanks to Mark Rosenstein, of www.apparent-wind.com/knots/ who painstakingly prepared the animation.
Knots and Splices Book of Knots RYA Knots, Splices and Ropework Handbook: G63 Knot Know-how: A New Approach to Mastering Knots and Splices