What is Clinton Anderson's method for trailer loading and why does it work?

Clinton Anderson's trailer loading approach is built entirely on the principle that the trailer should become the easiest, most comfortable place for a horse to be — and everywhere outside it should require work. The method starts before the horse ever sees the trailer. Anderson uses what he calls Downunder Horsemanship's approach to disengaging the hindquarters and yielding to pressure until the horse is soft, responsive, and looking to the handler for direction rather than thinking for itself. The actual loading process begins with Anderson sending the horse toward the trailer using rhythmic pressure from a flag or lead rope, then immediately releasing pressure the moment the horse moves even one step in the right direction. He never pulls the horse in. The horse is never dragged, and a handler never stands inside the trailer trying to coax with feed. Instead, he works the horse's feet — trotting or moving the horse with energy and purpose whenever it resists or backs away — making the area around the trailer the place where work happens. The trailer becomes rest. A critical element of Anderson's method is allowing the horse to back out of the trailer voluntarily at first, rather than trapping it. This removes the panic response that comes from a horse feeling cornered. Once the horse understands it can exit on its own terms, it stops treating the trailer like a trap and starts treating it like a stall. The process can take an hour the first time with a resistant horse, but Anderson's documented results show that horses trained this way load quietly and willingly long-term because they were never forced — they chose the trailer because it made sense to them.