In the video below, Arno Hendriks recounts his own early experiences as a rider and why he now has compassion for horses whose riders use bits. He then goes on to explain how bits work.
Bitted horses often open their mouths. Using models, this video demonstrates what a bit does and why the horse opens his mouth (to avoid contact pain.) When horses feel the constant threat of a bit on their palets while being ridden, they may adopt avoidance habits such as attempting to place the tongue over the bit, grabbing the bit, becoming fidgety, opening their mouths wider, or placing their chins on their chests.
In response to these avoidance behaviors, riders often don’t adjust their own habits and may simply resort to using stronger bits or devices that make it harder for a horse to relieve itself of pain.
When choosing to ride with a bit (and in many places you may not even have a choice as the majority of rental barns use bits), it is important to appreciate that a bit, no matter how “gentle” is still a foreign object being placed in a horse’s mouth and some will handle a bit better than others. Many equestrians say when used properly, a bit causes no discomfort for the horse. The problem is, what constitutes “properly?” Obviously, few, if any beginners are going to know how to properly use a bit, but are also not likely to ride safely without the control of a bit. But beginners aside, even advanced riders (as can be seen clearly all over YouTube) often violently yank and pull on reins clearly causing frustrated, high horses pain.
Even a single jointed snaffle, which many refer to as a “gentler” bit can hurt the soft pallet. The double jointed snaffle adds pressure to the side jaw bone. The jaw bone of a horse is sharp as a knife and applying pressure to push tissues (or on some horses, even the tongue) down along this sharp bone can cause excruciating pain.