What tool is placed on the upper lip of a horse to restrain it from moving while being treated?

The twitch is a tool specifically designed to help restrain a horse by applying a mild form of pressure to the upper lip. This pressure triggers a release of endorphins in the horse, creating a calming effect that can help keep the animal still during procedures or treatments. The twitch is typically a rod or clamp that can be squeezed to maintain the grip, allowing a handler to safely control the horse's movements for better safety and more effective treatment.

While halters are crucial for leading and controlling horses, they do not provide the specific focused restraint needed while the horse is undergoing treatment. A gag, which is generally used to keep a horse's mouth open, is not suitable for overall restraint of the horse's body and can lead to stress. The whispering bit, a gentler style of bit, also does not serve the purpose of physically restraining movement, as its primary function is to assist with communication during riding, rather than calming or restraining in a treatment setting.