Often I talk about the Conversation with the horse. This applies throughout any interaction with the horse. Recently I was asked about correct head postion.
Think of the head position as the "final" piece of the picture. It will reflect the quality of the engagement in the horse's hindquarters, his lifting of his spine through correct stomach muscles, allowing a lengthening of his topline from his tail to his whithers to poll, and finally he can soften his head and jaw into your hand.
The old saying is to ride the horse from the rear forward. But this requires time, patience and practice to correctly develop a horse's muscle development for balanced and soft self-carraige. Lateral work can be a tool if presented in a manner that doesn't cause the horse to be defensive. Haunches-in and shoulder-in can help him learn to independently move separate body parts, and develop muscles, in order to find the ideal movement without fear, flee or containment.
But most folks want instant gratification or quick "fixes," as you can see when you open any tack magazine. Many riders are imbalanced and unaware of correct aid usage to develop a horse, and they do not have independent aids, offering frequent unintentional and chaotic communication.
So instead they try to create a "pretty picture" with reins, leverage, and gadgets. They attempt to "yield" the horse's head into position without ever realizing that leveraging it in that manner causes the horse to hollow or drop his back, and his hind legs are "out behind" him.
You'll see the results of this type of riding with overdeveloped muscles behind the horse's ears and in the underside of his neck behind his jaw. You'll also noticed his front end been very muscular while his hindquarters show almost no development Depending on the frequency and severity of this approach, and the individual horse, it can eventually lead to lameness, arthritis in the spine and much more.
First thing to help develop the horse is learn to feel when and how the hind legs are moving. Then learn the timing and aids to influence the size and quality of the steps as they are the foundation for finding correct "head set."
Though this picture was intended regarding myofascia, it is a great visual for folks so see how influencing one area will affect elsewhere in the horse's body.