I know choosing/building leverage bits is a vast world of it's own.
But I have a couple of basic questions regarding the shape of the shanks on these bits.
1. The ones with elaborate S-Curves. How to those change the relationship between what the rider does with his hands and how the bit moves in the horse's mouth? I mean, the metal is stiff, so can't you just ignore the curve and just determine the angle of the bit's mouthpiece and the rein's ring?
2. Why do y'all want the shanks to curve backwards from the bit's mouth piece at all? Why not just build it "straight," like you'd see with an elevator bit. Is it just aesthetic-- because if you saw a bit build straight this way, the ends of the shanks would be sticking out ahead of the horse's chin.....life fugly boar's fangs?
And in general, what makes any of these stiff bits more or less "articulate" in the way they give or release according to what the rider's hand does? It seems to me, that by the time I put a leverage bit in Broke Machine's mouth, I want to talk to him in a very refined way.
Generally, a bit with a shank is used on a horse trained enough for the reins to be handled one handed.
Now, traditionally, those were curb bits with a straight mouth with some kind of tongue relief, from a very small rise to a spade bit.
Today, all bets are off, as many put shanks on mouthpieces with links, like snaffles used to be.
Whatever you do, try any bit on your own hand, holding the bridle by both cheek pieces in one hand above the other hand holding the bit with four fingers over it, horizontal to the floor, as if it was a horse's mouth holding it.
Then have someone handle the reins from behind and see how that can affect how the bit moves, the mechanics of several bits, depending on how the reins are used.
Always remember that bits are just one small part of what you guide a horse by, your weight and legs being a big part of how many horses learn to read what the rider wants.
Some riders get frustrated at times when they keep pulling on the reins and the horses are doing other than the rider thinks it is asking for, because the rider is thinking it's hands is directing the show, when it is their body giving who knows what signals the horse is really responding to.
The best school horses learn to differentiate when a beginner is learning to "talk" to them with their hands and to ignore the leg and weight signals.
Later, when the rider is more advanced, they can ride without reins, hands folded, learning to influence with it's body, as it acquires a more independent seat, one where the rider is with the horse more, not teetering up there so much now.
I think sometimes it's for aesthetics, but my understanding is that a bit that curves around the horses mouth tends to stay out of the way better for eating and drinking. If the shank curves back I would think you lose a small amount of leverage, as with a Grazing bit. When it just S Curves, measure the length of the shank "as the crow flies." The S does not add any leverage, just moves the shank away from the mouth. Cavalry shanks would have been shaped for eating and drinking, but when used today are mostly decorative or nostalgic.
Western Horseman Magazine has published a number of bit articles, series on what bits do with the shapes of shanks, mouthpieces, over the years. I found all the written stuff very informative, opened a whole new way of thinking to me. Maybe you could call the Western Horseman and ask about getting reprints of those bit articles. Some are kind of old articles, but the information is still good, have a lot of knowledge in them.
Western bits are a combination of tradition, what the rider needed to accomplish with the use of the horse, in what is chosen to use in the mouth. They also are QUITE willing to think in new ways, like the Mylers with that unique mouthpiece design they started with. Trainers needed a new way to communicate, so someone came up with a new method.
Reading some good books on bits will get you LOTS more answers than folks here trying to explain each and every design, mouthpiece, why it will or won't work well in THIS kind of riding, but not over there.
Some of the elaborate shank designs, and the shanks that curve back away from the mouth help keep the horse from lipping or grabbing the shank. Sometimes a busy horse will reach sideways with it's lips and play with the shank. Horses sometimes figure this out as a vice, where they will grab the shank in their mouth and then take off or do other naughty things, as you're essentially s.o.l. for control if pony has grabbed the shank.
Besides altering the balance, swept back shanks help reduce the speed of the leverage action. Straight shanks have a very quick action when the rein is applied, but swept back shanks are slower to act when pressure is applied and are considered a bit "gentler".
The best way to figure out what works best with your horse is to try out as many as you can. What your horse likes the most may seem weird to you. My advice is to just roll with it.