A twitch is a device that is used to restrain horses for various stressful situations, such as veterinary treatment. It is usually made up of a stick-like handle or a metal ring with either a loop of chain or rope on the end, which is wrapped around the upper lip of the horse and tightened. Another design, sometimes called a "humane" twitch, is a clamp that squeezes the lip with a nutcracker effect.
How the twitch works
The upper lip of the horse is grasped and the loop of chain or rope is placed around it, then the handle of the twitch is twisted until the loop is firmly around the lip. The twitch is popularly believed to work by distracting the horse, but may act instead by triggering the release of endorphins from the horse's brain, producing a calming effect. The twitch is considered a humane method of restraint and is commonly used by horsemen and veterinarians to keep an animal still and quiet.
Misuses of the twitch
The twitch is not intended for use on any other part of the body of the horse other than the upper lip. It may cause permanent damage and behavioral issues (such as a head-shy horse when used on the ear) if it is used incorrectly. Additionally, if a twitch is left on for an extended period of time, its effects will diminish. This may be because the endorphin levels of the horse are eventually expended, or because the lip goes numb.