Actually, it’s the shanks alone that make any bit a leverage bit. This is because of physics and the definition of torque which is Force applied multiplied by the length of the “lever arm”, in this case the shank. This is the same principle by which a “cheater bar” works to get lug nuts off a wheel when changing a tire. Increase the length of the lever arm and the same force results in greater torque.
The more severe the mouth piece, the more skilled the rider should be. It’s that old razor blade in the hand of a monkey thing. In skilled hands, it’s a highly sensitive communication between rider and horse.
The curb chain/strap functions to put pressure on the chin of the horse, basically discouraging them from opening their mouths to avoid the bit pressure on the roof of the mouth. The bit is rotated in the horse’s mouth when the rider pulls on the reins, with or without a curb.
There’s a great book out there on all the different types of bits and their uses. It old and it’s paperback with a yellow cover and I’m too lazy to walk to the other room to get the title. Snaffles are good for turning but not so great for stopping; curb bits are good for stopping, not so great for turning which I’m guessing helps us understand why Western riders neck rein.
As already noted by others, the vaquero tradition is very disciplined with respect to starting horses in a bosal and gradually educating the horse (and rider) to a full bridle horse.
For every person out there plugging away in a western saddle, I think there’s at least one in an English or dressage saddle equally educated or uneducated depending on your point of view.
I’ve spent considerable time in both worlds; AQHA lifetime member, over a dozen amateur all around and runner up awards, points in 8 events, USDF bronze and silver medals, all-breeds awards and year end awards; I’m still learning. The basics are, or should be, the same. Heck, even the German riding manual says all horses should be initially trained as dressage horses.