Non-Slip Loop Knot link:

[Bill Kiene semi-retired]

http://www.flyfisherman.com/skills/lkknots/index1.html

I was sent this link by Brandon Earhart.

This is the most popular loop knot today in fresh water and light salt water. Popularized by Lefty Kreh.

It does two things for you:

1) The fly can swing freely giving it better ation.

2) You can cast a heavy fly longer without knot fatigue from hinging so much.

[Digger]

So for sizes smaller than 12, I guess I'd stay with the 'Duncan Loop'

[Bill Kiene semi-retired]

Most don't use the Duncan loop anymore because it pulls shut on ever fish.

Most people are using the non-slip loop knot for everything but big shock tippet loops.

There they use the Homer Rhodes loop or Huffnagle loop.