I first saw this anchor in action at Smith Rock in central Oregon about 15 years ago. I was in the “mouth” on Monkey Face, belaying my partner on the final pitch. While waiting, a Canadian guide popped up behind me, and set up this anchor in about 15 seconds to fix the rope for his partner. “DANG”, I said, “That’s pretty slick, I've never seen an anchor built like that before.” He said it was common practice in Canada and the UK, where he had lived for a while.
This anchor is simply an overhand knot tied in about the middle of a sling. Each end of the sling then gets clipped to the anchor point, and a master point locker gets clipped to each loop made by the knot.
This is a sort of variation on a cordelette, with the advantage that it's easy to tie with a standard 2 foot / 60 cm / single length sling. Tying a standard cordelette style rig with a 60 cm sling usually doesn’t work, because the knot and loop take up too much of the sling material.
Another subtle advantage, as noted in the video link below, is that the overhand knot is much easier to untie after being loaded than a standard overhand loop.
As seen in the video, this is only used on a 2 piece anchor.
It’s completely redundant and non-extending, but like all statically equalized systems, does not fare so well in the load distribution / equalization department. However, if you have a well defined direction of pull, and all you've got to build your anchor is one shoulder length sling, this can be a good trick to have in the toolkit.
I'm sure this rig has some different names of different parts of the world, but I had never seen it named before watching the video below. Here, the guide calls it a “SWAMP” anchor, which stands for “Shelf Without A Master Point.” Catchy title, I like it!
Here’s an excellent video covering just about all aspects of building 2 piece anchors. The SWAMP is demonstrated starting at 11:10.
And, here’s a photo sequence showing how it’s done.
Clip a single runner to one bolt or gear placement.
Tie an overhand knot in about the middle of the sling.
Clip the sling to the second bolt or gear placement.
FInally, clip in the master point carabiner and rotate the knot upwards a bit so it doesn’t abrade on the rock.