The jointed mouthpiece which is considered mild as a snaffle may not be as mild when the shanks and curbstrap are added. Often, the milder curb bits tend to have a nice upward curve of the mouthpiece without being extreme. This allows room for the tongue without moving up into the palate as pressure is applied by the reins. The longer and straighter the shanks, the less pressure it takes from the rider to apply a lot of pressure to the various areas of the mouth and head. This article is designed to give a basic overview of some of the more common bits and how they act on the horse.
Many horses spend their entire lives, happily working in bits that appear very severe.
There are many other things to take into account when selecting a bit, probably the most important is the experience, balance and sensitivity of the rider. I hope this information has helped some gain a better understanding of how bits work.