I'm going to step away from Percy and his Hyena Project for a minute. But, everything is everything else, so I'm sure this will be relevant in any case.
I've been loosely following a friend's progress all winter as she worked with getting one of her horses to back onto a pedestal. Her blog is here. She's been amazing at videoing her progress almost daily- I will admit I have not watched all her videos and don't want my own experience related here to be anything other than "this is/was my experience".
Last winter I played a lot with getting Rumer to back in various situations. I still haven't actually hitched her to the cart because I'm too chicken to do it alone and don't have anyone to help me. But as part of the process, I wanted to teach her to back between the shafts...not that I'd necessarily ask her to do that for being hitched, but I thought it would be a good experience for her to be aware of them, be touched on both sides by them, be willing to back into them, etc.
I can't even remember how I started at this point, but I learned that trying to free shape backing when it involves an obstacle can be very difficult depending on how you set it up. First off is their eyesight. Most of us are familiar with that arc that is drawn around a horse which shows they can see everything except a tiny space in front of their nose and a larger pie slice right behind them. So unless you have a really wide obstacle behind them, they can't see it. Furthermore, horses are protective of their hind ends. They use their sense of smell and sight to check out everything they come across, then the front feet test it out. If the front feet decide it's not safe, the horse can either rock back on his haunches and lift himself off it, or push off with the hind end and leap over it. In both cases, the hind end is firmly attached to solid familiar ground. I found that Rumer was much more willing to back over or onto an object if she had JUST stepped over it going forward. I think in that case she knew what it was, where it was and that it was safe so she was comfortable stepping backward over it. So I would do a little circle of stepping over or onto something, then turn my body toward her as a cue to back (although initially this bit was shaped with guidance) and she would back over or onto it, then we would go back over it forward, circle around (taking turns left and right) and do it again.
When I say shaped with guidance, I mean that I stood still and clicked for movement backward but if she got out of line, such that she was backing but not in the right direction, I would either ask her to step over to get straight or we'd circle around and step over forward again to get her lined up right. True free shaping would have been if I'd sat in a chair with Rumer loose in an area with an obstacle, giving no hints and simply clicking for her backing toward the obstacle. Quite a feat that would have been!
So today I put three obstacles in the round pen: a rail (a short one, probably 4-5 feet long), a mat (plywood probably 2' x 3') and two cones (the tall pointy kind, spaced about 6 feet apart). I'm pretty sure she could see the rail and both cones when they were behind her because of the length/distance. The mat I'm quite sure she couldn't see so I put that along the fence as a guideline...she'd walk over it first and the fence was a landmark. Granted, it also physically prevented her from swinging in one direction, but not the other.
I set up a "training loop" which ended up including more than I originally intended. We've just lost our snow so I haven't done much with her in a while and she was very excited to have a training session again! This meant very forward marching right to all the obstacles, not exactly using her body well or being respectful of the person on the other end of the rope! I have to try hard not to collapse into giggles when little ponies are so determined. So we did a few reminder steps of bend-toward-me, step off to the outside and calm steps. I set it up as 3 steps like that and then C/T. Oh, with a verbal "walk on" to mean step forward. I use verbals a lot with her as I want her to be very familiar with them for work in harness (some day).
So the loop was: walk in 3 step increments to an obstacle, step over it, back over or onto it, then forward over it again to the next obstacle. The cones were in the middle of the round pen and I did a figure 8 switching directions when she backed through the cones. The clicks at first were after three steps, then I built to 6 steps. If there was an obstacle directly ahead, I created a chain whereby 3 or 6 steps was reinforced by the opportunity to go forward over an obstacle which was reinforced by the opportunity to back over an obstacle which was clicked and treated. I had checked first to be sure she still enjoyed both forward and back over an obstacle before using them as reinforcing behaviors. I also clicked for every step back at first to give her confidence but then dropped to either all four feet over the rail or through the cones or both hind feet on the mat.
3 (or 6) steps -> CT -> repeat to the ground rail -> step over rail -> I turn as cue to ask her to back -> she backs over rail with all 4 feet -> CT -> "walk on" -> 3 (or 6) steps bending left -> CT -> repeat to cones -> forward through cones -> turn toward her to ask for back -> she backs all four feet between cones -> CT as I switch sides so I'm now on her right -> "walk on" -> 3 (or 6) steps bending right -> CT -> repeat to mat -> she walks over mat (no stopping) -> and I turn to face her to cue back -> she backs two hind feet onto mat -> CT -> "walk on" ->3 (or 6) steps bending right -> repeat to cones where I switch sides and go back to the beginning at the ground rail.
She remembered quite well and in short order we were progressing around the obstacles smoothly. Only twice did she swing out of line. Once on the cones and once on the mat. Both times I was on her left and she swung her butt away from me. Both times I simply turned and walked forward (no click) on to the next obstacle. She didn't repeat the error. And when she did it on the mat, she swung her butt into the round pen panel. To me that means she wasn't afraid of hitting it and it didn't really physically prevent her from swinging out of line.