Fantastic Quick Knot for Small Flies - name unknown

A small, strong, and fast knot for all insect-sized flies. It wastes very little tippet but requires a long enough line to tie the knot, so it is not a good knot on a very short dropper line. It also makes a great and fast slip knot for tying a dropper on a hook bend.
Would someone please tell me the name of this knot if you know it?