This knot can really help climbers out in a jam. As it can be used to belay and abseil, you can either opt to finish your climb, or abseil down and collect your belay device when you drop it down the face of the crag. Maybe after you abseil down to get it, you might consider climbing back up the rope using a pair of prussic knots?
Thanks for taking a look.
Step 1: Video.
As always please feel free to ask me any questions if you are unsure.
Step 2: Method 1
Please refer to the pictures which I have captioned. I have done this as I think it makes it much easier to understand.