I also wanted to note a very funny little experiment I was playing with concerning rider balance:
Ever since I made the decision to change my posture due to tailbone related issues (IE tucking my pelvis/butt "under" is extremely painful) I have ridden with a more firmly "arched" back (but not inflexible) and chest/ribcage raised up out in front of me (as opposed to pulling my shoulder blades back, which creates stiffness). To me this harkens back to my days as a hunter competitor, though video of me riding like this I don't look as extreme as I feel.
My point is I discovered my balance, shock absorption, posting ability etc changed dramatically for the better!!!
I noticed that when I lose my balance while posting I was "fighting" to "pull" my froward-creeping legs backward by using my whole back and hamstrings... very painful and ineffective. In this new position all I had to do to regain my balance? Arch my back just a tad more - it automatically sent my leg back where it was supposed to be!
No pushing, pulling, fighting. In fact I found the various degrees of arch in my back helped me through many transitions, turns (sudden) and sudden random speed changes (Diego was one to play mind games with me that day). The real test will be to see how I sit the trot and canter in this saddle + position. I remember in my huntseat days your bottom generally "kissed" the saddle rather lightly at the canter, no heavy sitting. Very different from my dressage days.
I first heard of this position, back arched, rather than rounded, in the 1960'sm and introduced to it by an employee of the stable I ran. She had ridden at Sweetbriar, and was taught and coached by a contemporary, and I believe, collegue of Vladimir Littauer, the Russian cavalry officer that immigrated to the U.S.
Littauer had been exposed to the work of the Italian cavalry officer who studied how horses really move (in stead of going with the projection by humans of how they "thought," horses moved), Caprilli. Caprillie was so adament about what he learned and the experiments he was trying that he was sacked, that is removed from his position of authority. He soldiered on, and a few years later the entire Italian jumping team, using his methods, but with him abscent because of his dustup with his superiors, swept the field and stadium jumping events. They beat ever team in the world with what came to be called the Forward Seat, which evolved into the Hunt Seat.
Interestingly enough those that actually rode to hounds and "hunted," were among the last to actually adopt it with the English being the longest holdout and to this day has horse people that refuse to "ride like like and Italian monkey on a stick."
I have watched both the evolution of the forward seat in America and around the world, a short period actually, and the DE-evolving of it as well, as it has degenerated.
The place where it would save the most lives has abandoned it - the cross country portion of the Three Day Event, referred to as "Eventing," these days.
It takes guts to go over a drop off and actually be in the forward seat position as it appears to defy logic, but in fact follows the laws of physics and the true physiology of the horses' bio mechanics - and that of the human as well.
I took on a team, a junior team, of young women that wished to compete in 3 day, and after a few months of coaching and instruction I took them out one day, and rode with them to the top of a cliff - each of us was on a rehabed ex bronc that we had retrained and made into jumpers.
I had them all line up and told them, "It's no dishonor and you will not be judged if you choose not to follow me," and I rode off the cliff in forward seat. My horse slide down, with his hindlegs free to shift about and tucked under him providing directional control, while his forehand, with my extra weight there for speed control, basically helping hold him into the ground walked down the steep slope.
At the bottom I turned back and there they were, halfway down the cliff, every single one, in standard forward seat, head up, eyes forward, as I had taught, and every horse quietly handling himself down the cliff face.
My comment, before I turned and rode off to do some ditch and hedge work was, "Nice job."
I had, you see, a reputation with them, and the horse world round abouts, as a stern task master and I did not want them to see my smile or trears of pride welling up in my eyes.
First year their team took second over all, next year, a sweep of dressage, stadium jumping, and of course, cross country. Everything.
We called what you describe, with ones bottom just barely touching the saddle, "three point," as opposed to two point jumping position. I believe it's still referred to as three point. I teach it by asking for the crotch of the breeches to just touch the saddle, no body part.
And yes, we taught a flexible but arched back. I still do but have left all calling it "arched," as the devolving of correct forward seat has caused people to protest it as not appropriate and tending to stiffen. Nonsense.
One doesn't have to stiffen to arch the back a little. Unless forcing it. If we require a rider to be relaxed in all body parts they'll fall off the horse. It's a complex of muscle, tendon, joints, moving parts and position that is what "seat," is all about.
Some people cannot ride with the back even straight and must round slightly, others can be straight, while many, if they learn correctly, can arch slightly and be totally relaxed.
One of my students had trouble with getting any control at the trot with her lovely Anglo-Arab. One day, as I continued patiently correcting her seat suddenly she responded by doing precisely what I asked for - and her horse was transformed immediately from a reluctant hollow backed hind leg dragging nag, into the most beautiful full floating extension trot animal one could wish for.
I'm sure he had had some good training at some point in his life, because when he felt her seat conform to the classic seat, his eyes lit up, fire from his nostrils, a perfect ramener, and those beautiful legs extended and reach under him and drove him forward. A basket ball player could only wish for such breathtaking "hang time."
You are doing what is right for you. Don't worry about that rounded back thing.
Are you and Diego doing any jumping?
I suspect it, your seat, is perfect for Diego or you would not be getting such results.
Donald, Altea, and Bonnie Cupcake