Horse with a hollowed back
Unfortunately we see this head and neck-position quite often, especially with dressage horses. In this position the nuchal ligament and the muscular system of the neck have no effect. It's the muscular system of the back which is responsible for carrying the rider and therefore, is done with tension. That's why the connection to the hindquarters gets lost along with all its consequences.
„The most important thing is to detect the pure gait. This is the important criterion to decide if the horse is going with relative raising of the forehand or with a hollowed back.“
Translated from Erich Glahn