What I did with Blacky, is a combination of things. So pick and choose!
First of all, Blacky knows that when I start counting out loud, he should try to keep up that specific behavior longer. I taught him the counting when we taught him not to touch food if there wasn't a click first. First you accept when he only turns his head away one second, but then you start asking for a longer no-touching, which you can communicate very easily to your horse by starting to count out loud. The first x time you reward right after one, then when that goes well, you only reward when you've come to two etc.
When you start asking more seconds, sometimes your horse won't keep up the behavior (dive back to the food, get down from rearing, get up from the bow etc.), then you just ask the behavior again and start counting from one again. Very soon they learn that it's easier to hold that exercise than to start-stop-start all the time.
However, if he isn't physically able to hold the exercise longer, he will have no choice but to stop all the time after x seconds, so it's something you ('re forced to
) build up slowly!
So with Blacky's rear, I asked him to rear and then started counting and when he went down again I asked him to go up again, counted two seconds and rewarded. He can now do 4 to 5 seconds on a good day,
and I don't need him to stand much longer so we don't really train that for now.
Another thing, just like Romy experienced, is that a reeeeaaaaly slow rear strengthens your horse a dozen times as much as a quick tossing back the head and lifting the forelegs along. That's what Blacky did in the past, and it was quite ugly.
As I wanted him to do a collected pesade and not a hollow-backed rear, I started giving him lots of preparational signals with my body before I actually asked for the rear to go up. So indeed bending my knees , pointing to his hindlegs in order to tell him to bring them under his body, sagging further through my knees in order to tell the same to his hindlegs, and only then (after ten to twenty seconds of preparing like this) when Blacky looked really ready and collected in his hindquarters, I would lift my hand and ask for the pesade - and reward immediately for the slightest upwards lift! So no need to go all the way up for now, as the collection comes from a good start and that actually is the most tough part for the horse. When you click for collection like this, it's also an opening to the levade, which is only possible if your horse really has learned to be extremely conscious of his movements while lifting off, and strong enough to do the initial lift out of his pelvis, very collected.
I think I did that for quite a long time, the big preparation and only a small rear to top it off, in order to really get the rear to be a collected and conscious process, also because I was as much interested in the pesade and the levade!
However, you can't slow your preparation phase up too much, because at some point your horse will stand forwards so much with his hindlegs (collected), that he then really needs to lift off with his frontlegs, because otherwise he 'locks up' his frontlegs because he starts leaning on them too much. Then you get a mountain goat posture which is great in itself, but doesn't lead to a pesade that easily because most of the bodyweight is on the frontlegs then.
Now he really thinks about how to go up and does it way more collected (he has a nice pesade in the Youtube video on Haute Ecole) than before when it was very fast and hollow. And slowing the pesade down also gave us time to do things in that pesade, like the laufcourbette and the real jump courbette. When he still was in the hollow-backed-fling the head up phase, he simply didn't have any control over his hindfeet and wouldn't have been able to do anything more than just keep balance with them.
(And now I come to think of it, let's turn this into a sticky on rearing/pesade/levade!