I've read The Wild Trees and had been wondering myself how that Spider set-up worked. It sounds like Moss is describing a double lanyard set-up.
I have a double ended lanyard from Sherrill (11mm) with a 2-way mechanical cam in the middle that I use when climbing above my initial SRT rope. The cam is supposed to allow for single-handed adjustments of the lanyard length, while holding at that length if weighted. I keep the cam attached to my main center attachment on my harness, and alternate the other ends of the lanyard to the right and left side of my saddle as I climb, pulling slack through the mechanical cam as I go.
Is that the basic "spider" set-up? From the book it seemed more, I don't know, specialized than that.
My problems with this set-up have been:
1) I find the cam doesn't set as well as I'd hoped (it's called Gibbs CliMair (photo attached)). It is nice in that generally I can pull through the slack with one hand, but fully weighted the rope still has a tendency to slip through the device. If I'm stopped for a while, or strongly dependent on the attachment, I've had to tie a stopper knot in the rope to keep it from slipping through the cam.
2) If I'm suspended with the cam and side-attached lanyard, my saddle holds me in a funny position that's not real comfortable (compared to being suspended from only the center of the harness). So while it's good for tree security, you can't "hang out" with this type of set-up (at least, not with my saddles).
