I finally found a video to show more clearly what I was yammering about in the OP! 
Wolfy, I immediately tried this trick in a couple different applications, with various cordage. I'd only done this with eye-bolts and hooks before.
The only malfunction I had was using braided bailing twine, and old rope. The twine wasn't "slick" enough to slip smoothly. I tried it with some well worn cotton clothesline, also.
What happened was, when putting tension on the line, the double pass through the loop gripped the loop material and tried to roll it. The result was that the tag end wouldn't slip in the loop, so everything came to a stand still.
It works the dream with new, slippery poly cord however.