I couldnt figure out how to explain it properly, so here is how wiki explains it:
Pin firing is the treatment of an injury to a horse's leg, by burning, freezing, or dousing it with acid or caustic chemicals. This is supposed to induce a counter-irritation and speed and/or improve healing. This treatment is used more often on racehorses than on other performance horses. It is sometimes used in the treatment of bucked shins or splint, curb, or chronic bowed tendons. There was also the theory that it would "toughen" the leg of the horse. This treatment is prevalent in equine veterinary books published in the early 20th century; however many present-day veterinarians and horse owners consider it barbaric and a cruel form of treatment. It is not generally taught in veterinary schools today
My TB mare, Daisy, was pin fired back when she was on the track - still has little white dot scars all up the inside of her front cannon.
I've asked a few vets about it and from what they said, only people thought pin firing worked... they often used it on horses who would recover from their injury with just stall rest, so they pin fired and then gave the horse time off to recover from that, not the injury itself. While the horse was off for the pin fire wounds to heal, the splint/bucked shin/etc would heal also. Hence, people thought pin firing was the answer.
My Little Buddies TB Gelding, he has a tattoo so he was raced, and is 17 now, has these interesting marks on his front legs. It looks like someone took a pair of splint boots, that has nails in them and wrapped them around his cannon bone's.
I asked what that was, and they said it was something that happened to him on the track - what in the world is that? Pin Firing?
And yes, it is barbaric. I owned an OTTB who had pretty little dotted grid patterns across his front cannon bones from being pin fired for bucked shins.
I did also see it done at racing barns. I asked a vet why he agreed to do such a discredited procedure and he told me that if he didn't do it, race horse trainers would find an unlicensed or back yard person to do it; at least if he did the procedure it would be performed with aseptic technicque.