Ron Heinsman ( taiwanrock@geocities.com ) came up with this interesting knot that is a combination of a dogbone and a grapevine. Ron used it himself for a while but stopped out of concern for his safety when he realized nobody had tested the knot. I'm actually interested in using this knot myself so I sent a single sample to Chris Harmston of Black Diamond Equipment for pull-testing (thank you, Chris and BD!). As with all single samples, the results are not statistically significant and cannot be used to determine if this knot is truly safe.

Grapevine The standard way of tying a loop of cord is to use a grapevine knot (a.k.a. "fisherman's knot"). Overlap the ends of the loop and wrap them once, twice or three times to form a single, double or triple grapevine. Shown is a triple grapevine of Blue Water Titan 5.5mm Spectra cord. This sample failed at 3930 lbf (17.5kN). Point of failure was one of the 10mm pins.
Dogvine To make a dogvine, overlap the ends of the loop as you would normally, but also include the opposite side of the loop as you wrap the ends to form the knot. The resulting knot reduces the profile of loop in the middle and is potentially a compact way to sling chocks. Shown is the triple dogvine sample sent to Chris Harmston. This sample failed at 3476 lbf (15.5kN) at one of the pins.
Blue Water rates Titan Spectra at 15.1kN, so the two samples tested broke at completely acceptable strengths. And in other news, Tom Dunlap ("Tom the Tree Guy") reports that the dogvine is also known as the "sliding grapevine": I make my living as an arborist and have learned a lot of things from lurking on the rec.climbing page. I like to have new rigging and knots to use to make life easier. On your page you show the Dogvine knot. In August of 1997 I was a judge at the International Tree Climbing Championships. This is an Olympic type event for tree climbers. Not the loggers like you might see. We don't climb poles and cut logs with handsaws. The competition is made to simulate our work. Any ways, I was talking with the Norwegian Chapter Climbing Champion and I noticed that he had an interesting sling. He showed me how he tied it and was able to make a secured loop in the end. He told me that he had been using the knot for a while and just came up with it one day. Since then I have been showing it to many arborists and it is becoming quite popular. The name that we have come up with is a sliding grapvine. Dogvine is a good one too. The history of "inventing" knots is interesting. While your buddy may have stumbled onto the configuration a little while ago, I just wanted to share a little more history and show that climbers are thinking alike around the world and in other parts of the Vertical World. Tom Dunlap Canopy Tree Care WARNING: CLIMBING IS INHERENTLY DANGEROUS. THE OPINIONS AND DESIGNS PRESENTED HERE ARE MY OWN AND DO NOT CONSTITUTE A RECOMMENDATION OF OR ENDORSEMENT FOR ANY PARTICULAR OR GENERAL USE. IF YOU CHOOSE TO USE ANY DEVICE OR PROCEDURE PRESENTED HERE, YOU DO SO AT YOUR OWN RISK.