The conclusion so far for riders in a nut-shell:
1. Lift your head from C2, in the process stretching and straightening the vertebra.
2. Move back C8, in the process swinging the lumber area forward from L1, closing the shoulders and lifting the ribcage. The latter releases diapraghm and sides.
3. Gently lean your elbows against your hips, in the process creating a nerve impulse flow from the horse's mouth to L1, which is where - in the human vertebra - the spinal cord ends and the sensory-motor system transits into what is called the horse's tail.
So far so good. All this...
...is easily done on account of progressive reflexes, which seem to exist for just this purpose. However, when riding the modern horse, this progression and the rider’s resulting uprightness may not necessarily prompt the desired effect. Why? Because the modern horse is warped. Because it is longer and faster than it was before.
To compare please see a Roman equestrian statue dating to the first century A.D.