Pie and I have been struggling for a while with a couple of things - he tends to become disconnected between his front and his back end, resulting in "losing" the hind end (which means poor directional control and poor corners), and he struggles with softening, tending to "dive" with his head and neck which throws him onto the forehand, losing the hind end . . . The underlying problem was clearly (I'm slow, but even I get it eventually) relating to what the hind end was doing, and to how the hind end connected with the front end. These problems, as such things do, tended to show up more at the higher gaits - trot showed them up, and canter was even worse - he would tend to invert his head as well which showed that he was really on the forehand.
There was something basic missing. I finally had a theory that he didn't understand when I was wanting him to step under himself with the inner hind leg, to move his hindquarters over and keep them in line with the front end. If something basic like this is missing, I find that, rather than work on the places it shows up most at the higher gaits, I have to fix it starting from the beginning - in-hand and walk work. We did one entire session of just that, and then repeated some of that at the beginning of subsequent work sessions.
To start with, again as I've sometimes found it to be the case, it was necessary to exaggerate the cue - not necessarily make it bigger, but do it in a way that made it something different and notable for the horse. In Pie's case, this meant moving my hand (for in-hand work) or leg (for ridden work) way back - to the area of the back cinch. The fact that this was farther back than I wanted the cue to be in the end didn't matter - this was the "rough cut" which could be refined once he got the idea. I also didn't worry about other things falling apart in the interim - all I wanted was under with the inside hind, and maintaining forward - if his posture or head position were wrong, I didn't care - once he got the new concept, that would come back pretty easily. It was most important to isolate the one thing that was new so he could understand clearly what I wanted. I also made sure that a movement was established well in one direction before asking for it in the other direction.
We started in-hand with turns on the forehand - he knew how to do these already and there was no forward to have to worry about. I wanted him to take one step at a time off a soft cue with my hand. Once we had that, we moved to him making a small circle around me while stepping under and to the side with the inside hind - a circle where the front end was making a smaller circle than the hindquarters. Any time he started to lose forward, we carried on in a regular circle until that was reestablished and then went back to our stepping under. Once that was good, we moved to doing leg-yield and shoulder-in in hand to reinforce the inside hind leg stepping under and over.
Once that was good, we moved on to ridden work. Our first session was entirely at walk. We repeated everything we'd done in hand. I was still using the exaggerated leg cue to make everything very clear, and I continued to only worry about what the inside hind was doing, as well as forward, but nothing else. Pie being very smart picked it all up quickly - you could almost see the lightbulb over his head - "oh, that's what you mean".
Once we had walk cemented - it only took one session - we moved on to trot in our next session, after an in-hand and walk review. Everything stuck very well, and his corners were much improved. By the next session, I was able to start refining the leg cue, bringing it forward a bit - he still got it and with every session I was able to refine it a bit more.
Yesterday was the big pay-off. The ring was crowded, so we had to do some maneuvering - lots of turns and circles. Everything was going very well, with an even less exaggerated cue, so we took things up to canter. Bingo! The result of him being able to respond when I asked him to step under and and to the outside with his inside hind was that a number of previous problems evaporated. It was the best canter work he'd ever done, and under some of the most challenging conditions. His steering - my directional control - was automatic and easy - no wiggling of the front or back end. His corners and turns were exceptionally fine, even on what for him were very sharp turns and small circles to avoid other riders - previously he would have fallen out of canter on turns like this. And the softness and engagement from behind were there - no diving or inverting and the rhythm of the canter was much better - and these weren't even things I'd been directly working on. What happened was that, in addition to learning how to step under and to the outside with the inside hind, this also "activated" the inside hind, improving carriage and posture and gaving him the ability to engage his hindquarters and carry himself more softly. He was really lifting himself from the hind end and was connected from back to front, which improved everything else. He clearly got it and was very happy that I finally did. Pure magic!
Banging away at trot and canter wouldn't have fixed any of this. And riding the front end of the horse wouldn't have either. Getting him to move the inside hind under and over fixed a lot of things, but it was important to go back to basics to get it right.