First, two primary classifications of bits:
Bits that possess shanks. They utilize leverage to create pressure on the poll and under the chin (via a chin strap), and to also exert more pounds of pressure on the horse's mouth than a snaffle would. Each time you pull back on the reins of a curb bit, the pounds of pressure you use is amplified to the horse. The amount of pressure your horse feels is dependent upon the length of the shanks and thus the leverage applied.
In my honest opinion, I believe curb bits are misused much more often than snaffles. It's something called the "bigger bit theory": horse doesn't respond to a gentle bit, so it is put into something harsher, likely a curb bit. He doesn't listen to that, so he goes into an even harsher bit...and so the story continues until one day he's being ridden in a twisted wire single-jointed curb bit with 4 inch shanks. Finally, his owners have control. Or so they think. What people seem to forget is that this is a 1,000+lb animal. You are not going to out-muscle it, and relying on tools to do so will only land you in trouble - if not now, maybe a year down the road.
The only time a curb bit should be used is for refinement purposes, to allow for more intimate communication between horse and rider and to refine vertical flexion as collection progresses and is developed. If you lack control in a snaffle, get your horse into the gentlest bit possible (or even a plain rope hackamore if possible), and re-work the basics. Don't have control there? Work the basics from the ground first. Jonathan Field says "solve the problem on the outside, before going to the inside (the horse's mouth), so that you do not dull the inside", and I feel he is so right. I start any and all horses from the ground up: everyone starts out in a plain rope hackamore (after initially starting out with groundwork). Once I feel I have sufficient "control" (a.k.a. partnership between myself and the horse), I move them up into a snaffle, and finally (eventually), a leverage bit (if it fits with my purpose with that horse). For everyday riding, a horse should be in the gentlest bit possible, or no bit at all. Then you move them into a leverage bit for refinement, to work on more complex maneuvers that require a whole new level of subtle communication between horse and rider.
Bits that execute no leverage on the horse's mouth. This means it has no shanks. To clear this one up: snaffle bits are not bits that are broken, with curb bits being bits that are solid mouthpieces!! I'm shocked at the multitude of people who believe this to be the case. Either snaffles or curb bits may have solid or broken mouthpieces. Snaffles are your starter bits, used for the basics and for creating lateral flexion.
The following are some of the notes I took at the Red Deer Mane Event, from Jonathan Field and Bob Myler's demo concerning bits:
Bits do not train the horse! In other words, don't rely on a bit to "train" your horse: ie. don't upgrade to bigger bits when you feel like you lack control. Remedy the control issue without relying on equipment to do it for you.
There are two types of bits:
1- Those used for straightness (stiff bosal or a curb bit)
2- Those used for bend/lateral work/disengagement (snaffle/flexible rope hackamore)
There are numerous different areas in and outside of the horse's mouth that a bit can apply pressure:
- the bars of the horse's mouth
- the pallet
- the lips
- the tongue (the most invasive)
- the cheeks
- the chin
- the poll
Horses who periodically tip their nose up during sustained collection? Think about where your tongue sits when you are just sitting there on the couch, reading this blog. It's raised against the roof of your mouth, right? Well the same goes for horses. Many bits force the horse's tongue down, preventing him from lifting his tongue and swallowing. When a horse tips his nose up, he's likely just trying to swallow!
The angle of the spade on a spade bit determines pallet pressure and therefore headset (take note that in that particular headset then, the spade is applying no pressure). A spade bit is the ultimate level of communication, the ultimate level of intimacy, with a horse. It goes in only with prior and proper preparation, when horse and rider have achieved a high level of communication and training.
The reins are connected to the horse's feet, not the head.
Jonathan has a 5-step program he follows with his horses:
1 - rope halter for groundwork
2 - rope hackamore
3 - solid snaffle with a 1/4'' port ("lift") for some tongue relief
4 - solid snaffle with a 3/4'' lift for more tongue relief
5 - solid snaffle (can swivel around a barrel or such, but you don't want it folding in on itself so that it has a nutcracker effect on the tongue) with a 1'' lift
Keep in mind, a bit will typically not exert pallet pressure until the lift is about 2'' high. You start off with a smaller lift at first though and build bigger, because the higher the lift, the more pressure that is exerted on the bars of the mouth, though the more tongue relief. The lower lift assures that there is some tongue relief but that pressure is more evenly distributed as the horse becomes more accustomed to the action of that bit.
So, back to Equus:
The thicker the mouthpiece, the milder the bit (generally).
Single-jointed mouthpieces will exert a "nutcracker effect", pinching lips, tongue, and bars when pressure is applied to the reins. Double-jointed mouthpieces lack this nutcracker effect, however they exert more bar pressure.
The larger the ratio between the piece of shank above the mouthpiece and the shank hanging below the mouthpiece on a curb bit, the more leverage the bit will apply and thus the more severe the bit will be.
Smooth mouthpieces will exert less pressure/pressure points than a bit with bumps, knots, or twists in it.
On a snaffle, they serve to keep the bit in the horse's mouth. In my opinion, that is what a full-cheek snaffle is for on a green horse (after transitioning from a rope hackamore). If the bit is coming out of the horse's mouth, I think it's time to re-evaluate how you are asking the horse what you are asking. With a green horse in extreme circumstances, sometimes this is a little unavoidable, but for the most part if the horse is adequately prepared beforehand, the bit will not be sliding out of its mouth.
On a curb bit, the curb strap serves to exert chin pressure and allow for more leverage. Without it, a good chunk of the curb's action is eliminated. Make sure you can fit two fingers in between your horse's chin and the actual strap when fitting.
Twisted wire bits have NO place in a horse's mouth in my books, I really don't care the excuse used. Those are not designed for communication, they are specifically designed to hurt your supposed "partner".
There are more variations to bits than just the simple curb and snaffles, however those are the primary divisions...the rest I leave up to you to investigate.