You can see that the horse's exremely manipulated way of going makes it VERY uphill. the rider must be angled forward in order to not slide off the back of the horse (as someone else said). If a man does this, his "goods" will come into sharp contact with the saddle, therefore, I conjecture that he rounds his back to bring the "boys" off the saddle.
However, more importantly, in my mind, is WHY is this kind of riding ever liked? How is it even admirable? The way of moving is extrememly artificial. No horse moves like that normally. It can be nothing but very damaging to the horse's hocks, knees and in fact , all of his body, being jacked up on those platforms.
Why is this still practised? It has to be THE ugliest form of horsemanship I have ever seen. I know this will really anger many of you, but I will just have to speak frankly here.