Take a dressage whip and curve it, making an arc. The arc is strong and flexible. It can bear pressure without collapsing. Straighten the whip and discover that the straight line is flexible but weak. It cannot bear pressure without collapsing.
The desire to have the horse in a natural posture is only good when his back is in a natural state, unencumbered by the weight of a rider.
No matter how natural we want to be, the act of riding a horse is unnatural. They are designed to pull, not carry.
In this position the horse is rounded from croup to poll and his back lifts to carry the weight of his rider.
This is not a forced position. It's not about pulling in the head and driving up the hindquarters.
The clinician we saw was riding a gorgeous Andalusian stallion. He had a double bridle and was being ridden on contact. Yet when the rider moved his hands forward, the horse maintained his frame and his cadence. A beautiful example of self carriage.
I don't know that I have what it takes - either by my skill or the ability of my horses - to achieve such magnificence. But more and more I'm convinced that ignoring the physical - failure to help the horse move in a bio-mechanically correct manner - is just as detrimental as ignoring the mental and emotional.