The correct riding position on a horse did not just come about. Riding a horse started a few hundred years before Christ. We used to keep horses before, but mainly as pack and draft animals. Before that we only ate them. Since we started riding, there have been publications on how best to do that. Since the horse was a means of transport for a long time, it had to be done as efficiently as possible. Otherwise it could not be sustained. Moreover, it had to be a position in which you could give a horse signals that he could easily understand.
The position we use to this day in dressage is centuries old and it is always important to make an effort to sit on a horse that way. Why? Just try running around with a child on your neck. If it moves all over the place or leans to one side with its arms flying around, you will have the greatest difficulty keeping your balance and walking in a straight line. The same applies to your horse.
If the prescribed dressage position apparently is the most efficient, then why is it so terribly difficult to sit like this with your back neatly straight and your legs quiet? You have to practice everything you want to be good at. The right muscles need to get stronger and be more elastic. Your mind controls your body via nerve impulses. Picture those nerve pathways as a road network. What you often do becomes a road you use a lot, like a highway. At first you have to think consciously to perform that action, but after a lot of repetition it is more or less automatic. A movement that you do less often becomes a sand path. It is slower, you have to put in more effort.
In the old days people sat on a horse for a long time to get from A to B or to go to war, giving plenty of time for repetitions from your brain to your limbs. Nowadays, we rush to the stable after school or work for at most an hour of training. No wonder it is not so easy. But it is worth paying a lot of attention to it.
Like a bag of potatoes
I drive my students crazy, because I always hammer on about posture and seat. I think it is super important. How can you expect a horse to perform all kinds of complex movements if you lean to one side and sit like a bag of potatoes? How can you give your aids lightly and correctly when you have no control over your body? Are you really sure that you don’t accidentally do something with your hands when you give a leg aid? The intention is that you have an “independent seat”. Someone once described it as cycling with free hands. I think that’s a nice explanation, except that elastic light contact is also necessary for communication.