One of the reasons it’s so important to not over manage the horse’s head and neck or to try and create roundness through the reins is that the horse’s head and neck offer us valuable information about the body. If the horse throws his head up, that’s a good indicator his back has hollowed and his weight has come onto the forehand. If he’s counterbent, he’s likely fallen in with his inside shoulder and rib cage. Trying to fix these imbalances with the reins forces the horse to crunch their neck into their shoulders and further imbalances them. If, however, you fix the body, the neck and head will fix too.
Try it yourself : standing in a normal, relaxed posture, drop your right shoulder down so the space between your rib cage and pelvis is squished. What happened to your neck and head? You’ll probably notice your left ear is now higher than the right, and your chin tipped slightly to the left. You are “counter bent.” Now, without changing anything, turn your chin to the right, keeping your ears Unlevel. You probably feel pretty uncomfortable, and maybe stopped breathing as well. This is the equivalent of fixing a counter bend with your reins.
Now, shake it out and try again – drop your right shoulder and you’ll find your left ear come up again. To fix it, take a nice breath in, and lengthen the space between your right side rib cage and pelvis. You’ll notice your ears are level again, and your shoulders are too. No need to fix the head and neck.
Ride the horse’s body, and the head and neck take care of themselves. Ride the head and neck, and you have to fight with the body of the horse.