From what I know, the original thought process of firing was -- Ok - it's hurt, let's hurt it more and encourage more healing activity in the area to fix the original problem faster. On the track, it's all about time and money -- time IS money. If your track horse is injured, it's not earning money, it's costing -- big time. So it needs to be fixed as fast as possible. So more hurt = more of the horses energy spent on healing in one particular area so it will heal faster, right?. The really stupid thing is that with or without the firing, the horse needs to be pulled from that year's races. Period. Now, to that 6 - 12 mos off for recovery of the initial injury, let's add another injury -- the pin-firing for the horse to recover from. Sometimes, so I've been told, trainers will accept that the horse needs to be pulled for treatment and the pin-firing, but won't agree just to pull the horse for the initial injury. So they spend more money, more time, more drugs to do more damage. Some people say that the scar tissue that results from the pin-firing strengthens the damaged tissue further, by providing scar tissue support around the damage. The problem with that thinking is that scar tissue doesn't flex and pin-firing WILL result in scar tissue on top of any scar tissue from the initial injury. That's why my girl Lisa will always have "bent" legs. Her legs are severely scarred from the initial bow and then the firing on top. You wouldn't believe the ultra-sound pictures. The damage has completely changed the actual structure of the soft tissue due to scarring. Instead of clean lines of tissue, it's cross-hatched, dense and stiff. It's like looking at a pastel drawing -- all blurry.
Lisa is walking, living proof that pin-firing does more damage and no good at all. Her tendons are hard to the touch. She is no longer in any pain at all, but she would never be able to be a competitive horse because the flexion just isn't there anymore.
You don't need to worry about the marks your horse has. These are superficial injury scars, IMO.