David_T, .....what your boy shows in "More Pics" is similar to what I went FROM to get where I am now. Differences are these:
1/ I start out with the sling, etc more to the side of my head, ....much like the position you'd use in drawing a bow, if you took the bow string somewhat past your ear. I stand with my side toward the target, and a line thru my toes running thru the target.
2/ I put in 1 FULL additional rotation before launch. This is what made the huge difference from where I was to where I am. Total rotation is 1-3/4turns. It looks like your boy is putting in between 3/4 and 1 turn.
3/ All rotation is in a vertical plane, with (for now) almost all arm motion in the forearm and wrist only. Maybe I'll figure out how to get some shoulder power into the launch later. For now, anything other than forearm & wrist lofts the rock and puts a bad "hook into the drive". Standing like I do, all (most?) rotation is in front of my body, not off to the side, though I think I twist my body a little on launch. Once I get the rock moving, I think my elbow is pretty much "parked" near my hip.
4/ Start of the rotation is relatively gentle, with (I think) some "whipping" acceleration right before launch. I wish I could describe this better. But for sure!, if I try to "force" the acceleration, I mess up accuracy big time, AND take a LOT of launch velocity off the rock. The whole spin-up has to feel "natural" and un-forced to work right for me. I figured this out when I noticed that gentle warm up shots were flying a lot faster, straighter, longer, lower than later "muscle" shots.
So, slinging is starting (for me) to feel a lot like golf with importance of proper setup and form, easy swing, let the club do the work, etc.
I've used x2 slings, ....one of Ben's Peruvian models which measures 33" center-of-pocket to center-of-finger-loop of the retained cord, and a homebrew item just under 37" measured the same. For now, the Peruvian is working much better, and I think it's because the retained cord is relatively fat and rough, and has a "more secure feel". The homebrew sling cords are slick hard 1/8" braided nylon, and have a lousey feel.
The movie idea is a great one, especially if we could run them in slo-mo. I'd like to see if I'm really doing what I described above.
mgreenfield