Trailer loading is one of the most revealing practical tests of a horse's yielding education, because it combines forward pressure response — the horse must move forward into a small, enclosed space — with the horse's natural claustrophobia and the handler's typically elevated stress level when the trailer needs to be loaded and the horse isn't cooperating. Every element of the challenge is a direct application of yielding to pressure, and horses whose yielding education is solid consistently load more easily than those for whom yielding is only a theoretical understanding.
The specific yielding skills that trailer loading requires are: forward response to lead rope pressure — the horse moves forward when asked rather than planting or pulling back; yielding through concern — the horse moves forward even when it is uncertain or mildly worried about the space ahead rather than shutting down when the situation is novel; and patience when stopped — the horse stands quietly inside the trailer once loaded rather than scrambling to exit as soon as the pressure of loading is removed.
The approach to a horse that refuses to load is almost always the same approach as for any other pressure-and-release problem: apply the pressure (forward on the lead rope, driving energy from behind, or a combination), maintain it through wrong answers (backing up, planting, spinning), and release completely and immediately the instant the horse takes any step toward the trailer — even a step toward it rather than into it is a correct response that earns a release in the early stages.
Clinton Anderson's approach to trailer loading explicitly frames it as a yielding exercise rather than a trailer problem — the horse is not scared of the trailer, it is choosing not to yield to forward pressure in that specific context, and the solution is the same as the solution to any other failure to yield: clearer, more consistent pressure with precisely timed release on the correct response. Horses that have been through this process systematically load from a light rope feel without hesitation, because they have learned that forward movement is always the response that produces relief, regardless of what is in front of them.