Despite all I've read about how to position oneself in the saddle, and despite my best effort to follow advice, it is still easy to mis-understand, and easier still to slip into poor posture.
So, re-reading the advice for beginners in the EC handbook, and re-reading Sally Swift's definitive work, and discussing it with my coach while doing my usual, strenuous riding without stirrups, we came to two observations, viz., that the stirrup bars and flaps of the standard European riding saddle are too far forward, either the General Purpose ones, and especially the Jumping saddles, and that Dressage saddles are probably too far forward as well. All of which encourages the legs to be too far forward.
I'm not sitting in a chair, but that's how the majority of riders (me included) tend to sit. So even without stirrups, I'm still just slightly behind the centre of gravity of my horse.
I moved my knees back, about on hand-width, to the very back of the flap, and dropped my legs down.
The change was amazing. The result, was my pelvis rolled forward, so that the front was in contact with the base of the pommel (not really all that comfortable for guys, btw), and both bones at the back, in the middle of the saddle. I suddenly found myself perfectly balanced, as described by Swift. I was effortlessly balanced over my pelvis, my legs gripped almost without tension, my weight right over Oakley's, Oakley in balance on the ground.
Oakley suddenly picked right up. Riding, even posting, without stirrups felt vastly easier, Oakley was in a frame, hindquarters tucked under him, his trot flowed, and even jumping over low obstacles, about 40cm (1ft 3in) was easy. So was the release. My torso became relaxed and upright, chest out, shoulders back and down, as if my scapula were being pulled down my back on their own. Everything came together beautifully. Usually I add stirrups after about 20 minutes because my legs are so tired, but with that subtle change of posture, an hour went by with no need of any assistance. When I did put my feet into the stirrups, I had a hard time keeping my balance again.
Again, it's one of those subtle points that can only be learned from experience, no amount of writing can properly describe, and is best learned from someone observing from the ground. But once learned, it's like the curtains being pulled back on a sunny morning.