I was worried about it slipping the first few times I tryed it too. That's why I started using 3 strand at first, thinking it might hold better, but its not necessary. You still have to get it more or less centered. I keep slack in the rope that goes through the eight, where it goes around the big ring on the bottom, pulling both ends from the middle. Then when I tighten the rope it self centers.
I don't use it that often, but the times I do, it works perfect.
I got the idea from watching a self leveling device a friend designed for use on his crane. On the crane a large limb cradled like that
might only move an inch from the position it was cut at.