Your reply here is pretty much "industry standard" for dealing with a pacy horse. It's pretty much what I was told back in 1990. It was in 1991 that I "got religion" and rejected the "industry standard" for something that worked better, cost less, but did require more time (which I had).
Note that I was preparing the horse for the field, not the show ring. My goal was maximized efficiency of motion not any sort of "style."
Regarding angles, if you were to stand up 1000 horses of any given breed you could come up with some means and averages on lots of bio-metrics (hoof angles, leg angulation, back length, etc.). That does not mean, however, that any given animal will have the number that is derived from a statistical analysis. Would that number be a good starting place? Maybe so, but maybe not. Horses are not pieces of standardized plywood or steel; templates with them are of very limited utility. The trim to anatomical correctness will always be right for the horse in front of you. That's not a "template"; it's reality.
I'm trying to understand the concept of a snaffle being more harsh than a curb. Each is made of metal, each rests in about the same place in the mouth, each has a range of effectiveness in some form of communication. The idea that one is more "harsh" does not compute. If anything, the curb has more power (for good and for ill) as it is a lever. I remember my Archimedes.
Just where you use each can vary with discipline involved. In the show ring, where presentation is everything, the curb will likely be the choice because of the arbitrary standards that the judge will apply to the exhibitors. In the field, where performance is primary, it might be the choice because of the horse's temperament or discipline.
The standard for the Army in the early 20th Century was the bit and bradoon of the full double bridle. Recruits began training in this rig from day 1 of boot camp. Their horses were old troopers and did not need the bit; the bradoon was sufficient. Both were used, however, because the combination helped the trooper develop a proper feel for the horse's mouth. In modern times Helen Crabtree, in her book Saddleseat Equitation
, makes the same recommendation. When the Army went after Villa during the Punitive Expedition the bit was left in garrison and most officers and men rode in the bradoon, only. This was to be the standard (bit and bradoon in garrison for training, parades, etc.; bradoon in the field) until the end of the horse cavalry in 1948. I think is probably a pretty good standard for most riders, gaiting or trotting, today.
Can "action devices" be successfully used as a "bridge" to get from where you are to where you want to be? Probably not. I guess there might be the one horse in a thousand that might
be trained to a certain movement with a device and then continue that movement after the device is removed. I've yet to meet it.
To me "action devices" and cattle prods have a lot in common. If you want to teach a horse to move on word or a touch then combine that word or touch with a shock from the cattle prod and you'll get a much quicker (albeit more dramatic) response from the horse. It might even have some "residual memory" (although "action devices" generally only work while actually applied).
They are also "anti-training" devices. If you use a crop or a spur to induce some movement when the movement is attained you stop using the crop or spur. That's training. An action device is always "on." If the horse is moving incorrectly the action device is "on"; if it moves perfectly the action device is "on." The rider can't turn it "off."
Can they be "effective"? Very occasionally yes, but most of the time "no." And they quickly become the province of "quick fix artist" who gets a "result" but does not lay down a foundation for that result.
For these, and other, reasons I reject them.
I shudder at the use of the word "natural" when combined with "horsemanship." It's an ultimate "oxymoron." As to the farrier's theory, I say no. Chains only work while they are on the horse. If it were otherwise they would be used only in the training barn and not in the show ring.
Still, the really good horsemen of history have understood the natural proclivities of the horse and worked with them to accomplish their goals.