It's Earth Day, But I Had It First
Yes, it's my birthday and the entire world celebrates with Earth Day....just kidding, of course...but I was born long before Earth Day was so named. The only plus is that I am finally going to collect Social Security. And, I am going to have shrimp for dinner.
Muriel asked about how I teach "on the bit." So much of it is establishing a "feel" for the horse and when to hold and when to let go...sort of.
The first thing that must be in place from the horse is forward. This does not mean running or going fast, but rather a willingness to move with energy, particularly from the hind end. If the horse is not using his hind legs to offer to carry his body, then there will not be a connection to the rein.
Once they are going forward, most horses will tend to lower their heads a little and start to relax their backs. Actually, I don't think a horse can honestly be forward unless it is using its back. When I first got my PJ, he had been a race horse, but I seriously doubt he'd ever reached his potential for speed because he had never learned to go into the bit. He "galloped" with his head up a little and a sense of "disconnect" back to front. It's just as well for me, because once he learned how to engage his hind end he was by far the fastest horse in every group I every rode with. I have a feeling he would have been incredible on the track.
Once forward is established, the bit comes into play. The idea is to create an elastic contact on the rein. As Muriel noted, suppleness comes into play here. Horses that are stiff side to side, need to become soft laterally. Exercises to loosen them include movements that "displace" hind legs, front legs, shoulders, etc. encouraging them to be flexible. However, some horses are already laterally flexible and can use sideways movements as evasions, so those horses need to be made longitudinally flexible instead--lots of transitions help here.
Once the horse has suppleness, creating the "on the bit" feel becomes a matter of "feel." The outside rein is the key here. And with it, the half halt. Ideally, the half halt first comes from the rider's seat where, instead of following the horse's motion, the rider resists very slightly. The rider's hand closes on the outside rein, just enough to get the horse to react by "thinking about stopping." But, at the same time, the horse must also continue to go forward. I usually apply a little leg at this point to say, "Keep stepping forward, but don't take such a big stride." (Instead, kind of step "up" under yourself.)
At this point, we hope the horse will step under its body more with the hind leg, thus starting to carry more weight on the hind leg and, in the process round its back under the rider's seat. Also, in response to the rounding back, the horse will arch its neck a little and soften to the rein.
All this happens very quickly and once it does--even for a stride--the rider needs to soften as soon as the horse does.
Now, this is tricky. First, softening does not mean a complete surrender of the rein (that's another exercise where you teach the horse to "give to the bit") but rather a maintaining of a light, elastic contact the the horse's mouth. Releasing the rein altogether and then half halting again, as you will probably need to do, hits the horse's mouth, acting as a punishment.
What you need to do is ride "from contact into contact," always keeping a slight tension on the rein--which might vary from horse to horse--and decreasing or increasing that tension in the half halts.
During a ride, as the horse is being trained, a rider might have to half halt every few strides. As the horse learns to carry itself more and more on its own, the half halts are needed less and less. Even the top dressage horses still need half halts as preparations for various exercises. The piaffe is kind of the ultimate "long" half halt and passage is a half halt in motion.
Now, mind you, I have never ridden a really talented dressage horse, but I suspect that a horse bred and well built for dressage makes all of this much easier than a horse with much less natural self-carriage and balance than my horses. I can make my horses soft in a lower, longer frame without too much trouble, but asking them to "balance" back onto their hind ends for some of the upper level exercises does become a chore. It's the reason I retired Toby and why now, I try to be careful with Tucker. Neither one is physically built to easily work in a upper level balance and so getting them there required a lot of strong, repeated half halts which was difficult for them and for me. The consequences were some hind end soreness on their part which if much more common in dressage horses than many people realize.
I believe that a horse's ability to carry itself on the bit is very much affected by conformation. We can certainly teach just about every horse to go on the bit and to round its back. As a matter of fact, most horses I've "met" that did not round their backs at first seem to really like it once they discover how to do it. If you arch your own back by leaning back, you will feel a tightness of muscle and joint. Now imagine a heavy weight (a rider) sitting on that arch. Pretty uncomfortable, right? Now, bend over at the waist instead and you will feel your own muscles stretch and relax. Imagine the weight there this time, and you will realize you can carry it much more comfortably. So it is for the horse. Lifting and rounding his back under a rider is a far nicer way to go.
And :that's what the goal is--to make it better for the horse.