I was recently viewing the Myler DVD A Whole Bit Better (which for now is $9.99 and includes a copy of the book from www.toklat.com). This is an AWESOME video for anyone who wants to learn more about bits and how they work. The purpose of this video is to sell you Myler bits (which I personally like and use), but even more than that, it has great information.
There are six points of pressure that can be caused by a bit.
1. Tounge pressure. Either the bit pinches or restricts the tounge
2. Bar pressure. This is the area of the horses mouth between the teeth.
3. Curb pressure. The 'chin' of the horse, underneath their mouth.
4. Poll pressure. The pole is where the head pivots from the neck
5. Palate pressure. This is when the bit touches the roof of the mouth.
6. Nose pressure. Pressure on the nose, usually from a bit that combines nose pressure with some type of bit action or can be without a bit.
Different types of pressure can create different results. Don't forget, the bit you use is not just for your discipline and definately not a substitue for good training.
I can ride all my horses in a rope halter, but the bit is about refinement and communication. And as my friend Gina Gardner says 'The horse chooses the bit'. So true, so true.