It is just easier to copy/paste from past responses, so here we go.
Some are hand stretched, some are in "humane braces" where the high tail is just a piece of wire and a "cap" or wig, but most of them require a simple surgical procedure. They are put in a harness that is specifically fit to each horse. At first, several weeks before the surgery, they are put in a bustle which is a really thick crupper to help loosen the muscles and to ensure that the horse will even tolerate wearing the set(You don't want to find this out the day after surgery). After a few weeks of wearing the bustle and hand stretching, a vet comes out and does a simple procedure where the muscles that allow the horse to clamp their tail down with force are nicked. When they heal, the horse still has full use of his tail, they just cannot clamp down with force. It also makes the tail loose enough to be put in a brace for the show ring creating that aforementioned balanced look.
It is purely for fashion now. Not necessary, and they require an absurd amount of care during healing. Every effort is made before, during, and after the surgery to make sure the horse is comfortable and pain free in order to protect the tail. A horse in discomfort will rub his tail, pull his set off, of find some othe way to try to rid himself of the discomfort which will obliterate the tail and cause the horse greater pain. When this happens, often the damage is irreparable. They are pampered, and carefully tended every day until they are fully healed.
Once the procedure is complete, the horse does not need to wear his set 24/7. Show horses only wear their sets a week or two before a show. In the off season, the sets come off. So do the shoes.
It is legal in every division to show with a natural tail, and some horse shows, including the World Championships have added several natural tail classes.
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I don't know what barn you are at or where this garbage came from, but tailsets on a set horse are not pulled during 'off season', and darn sure don't just wear them week or two before show either. The only time they won't be wearing them if they are a set horse is when being worked.
And it is a tailset not a harness.
And most of them will do just fine being started with bustle and moved up into crupper, no need to cut one.
As for not being able to clamp down? I can assure you that the few horses that had had to have tail worked on could clamp them down just fine and dandy.