How does Clinton Anderson's Desensitize and Sensitize framework work and why are both halves necessary?

Clinton Anderson's Desensitize and Sensitize framework is one of the most conceptually important structures in his Downunder Horsemanship program because it addresses both halves of what makes a horse safe and useful — and he teaches that most horse owners do only one half, which produces a horse that is either dull or spooky depending on which half was neglected. Desensitization is teaching the horse not to react to things it should not react to — the tarp, the flag, the plastic bag, the sudden noise. This is the half most people think of when they think of desensitization. A well-desensitized horse stands quietly while objects move around it, sounds happen near it, and unusual things touch it. This horse is safe because its flight response threshold has been raised. Sensitization is teaching the horse to respond immediately and precisely to the handler's aids — to move forward from a light leg, to yield a hindquarter from a light touch, to move laterally from a slight shift of pressure. This is the half most people skip in favor of desensitization. Without it, a horse that does not spook is still not a safe horse if it also does not respond to the rider's requests in a moment of need. Anderson teaches that both must be developed in balance and that they reinforce each other. A horse that responds instantly to the rider's directional aids is safer in a spooky situation because the rider has control of the horse's feet even when the horse's attention is on a frightening stimulus. A horse that is desensitized is easier to sensitize because it is not dividing its attention between training and environmental concerns. The training implication is that every session should include both — some work that builds the horse's confidence with the environment and some work that sharpens the horse's responsiveness to the handler's communication.