You don't need a horse.
The way to start developing the leg for riding up off the saddle in huntseat position doesn't require, and in fact should not include, a horse.
Use a step, or a piece of 2 x 4 wood about two foot long.
Stand on the board, or the edge of a step, steadying yourself with your hand or hands on whatever is nearby: a chair back, the wall, a stair railing.
Only the balls of your feet should be elevated on the step or board. Knees slightly bent.
Feet about a foot apart. Toes at first pointing straight forward. Let your weight sink down into your heels by relaxing your calf muscles.
It helps for some to find that point of bend of the knees where the calf will best relax.
You may want to actually rise and lower yourself using your foot and calf muscle and as you come down release and relax the muscle.
DO NOT BOUNCE. If your Achilles is shortened you could experience a sprain. As in all stretching exercises practice the Yoga rule: push only as hard as is comfortable, and not to the point of pain.
At first just a couple of minutes is sufficient.
Fairly soon, even the first session if you are comfortable, let your hips rotate until your back is hollow. When in this position on the horse, later, barely any or no weight should be on the saddle.
A few minutes a day will stretch the tendons. You'll know you are there when, with heel fully down, you can relax your calf muscle.
It will feel stretched but not tensely uncomfortable as it likely will at first.
While you can try from the first to lift your toe you'll likely find that difficult. Don't sweat it. That will come in time as an exercise.
Begin, from the first time, if you can, to rotate your ankle bone inward -- toward each other. You may find you can do it more easily if you shift your foot forward or backward on the board or stair you are standing on.
Needless to say, at some point you want to be wearing the footgear that you ride with. Like your leg the boot much be broken in to the angles used in this position.
If you are flexible, or gain sufficient flexibility through this exercise, a 2 x 4 will no longer give you sufficient height. Move to a stair step.
When your heels can drop a full 2 inches and you can relax into the posture I've described you are likely ready to ride.
Use your hands on the horse's neck while you find your balance. Practice at the walk for a week or so before trotting. Trotting, of course, will give you the next step in the exercise ... and will add what I asked you not to do at first ... bounce.
If you can follow this in your mind visually, it will become apparent that you have created a pair of shock absorbers not unlike those in a car.
Instead of your thighs taking up the impact of trotting, or cantering, (don't be in a big hurry for the canter), your lower leg absorbs the shock with a nice spring. Next to your thigh your calf carries the largest muscle group in your body. And your knee is built to flex like a spring. And so is your ankle.
I may, if I can put the parts all together at some point, make a video of this. We'll see.
A review of the Web shows exactly what I saw 30-40 years ago ... they (the instructors) all cry out to the student, "heels down," but fail to recognize that is only part of the paradigm.
Putting one's heels down has almost no effect on the 'seat.'
It is heels down and ankles rolled in that gives stability. The leg, from the knee down now becomes the platform upon which one rides.
In fact, heels down, alone, can well cause the knees to rise from the rider pinching with the knees and thigh.
In fact, usually, when an instructor orders "knees in" before heels down/ankle cocked is mastered are asking for the student to pinch themselves up off the horse, whether or not they realize it.
Some students, of a particular body type, leg shape, flexibility can do it properly, but hte majority of us cannot. Not until we have the ankle control mastered.
Heels down, alone, still allows the rider to sit on their thighs, rather than stretch the thighs down around the horse.
I wish I knew and better understand the mechanics when the ankle is rolled inward, nevertheless I can feel it, and so could every one of my students.
As Glen is experiencing, there is a very different feeling when this is mastered.
Here's a simple picture of the direction of applied force. Note the toe is out. Some judges will fault this. But it's simply a matter of anatomy. We are not all made the same. If the ankle is cocked over the downward pointing heel, the desired springy platform is attained:
I can tell that, in the above image, at the instant the photo was snapped the rider did not have their weight down in their heel, or this person has very stiff tendons ... something a lot of us have. I have a bad right side Achilles so I know the feeling.
I doubt the rider is in 'two point,' at the time of the photo.
The sole of the boot, by the way, is in perfect position ... that is the sole planted against the inside stirrup upright bar. This makes the tilt of the ankle far more easy, and stable.
And I am not the only person teaching this, though I see zero mention of how to prepare the student for this. They do it on the horse. Something I don't care to do, since the beginner is going to wobble and if they have reins in their hands, oh my.
I know that many instructors do know about this and do teach this, but not enough of them.
This is turning into a book. Sorry.
Enough of my rambling. We'll get back to this when you wish.
Donald R.