Twitching a horse’s upper lip is a very useful technique for restraint but it should only be used when treatment without a twitch cannot be done. It can be helpful to allow treatment of painful wounds and other short procedures. The lip twitch has been shown to cause an endorphin release in horses. Endorphins are natural painkillers that exert a calming effect.
However, a twitch does not work on all horses and can be dangerous if misused or used on the wrong horse. Depending on the horse, a twitch may only work for a few minutes before resistance increases. In my experience, twitches work pretty consistently on donkeys and mules.
In my opinion, the use of a twitch is not inherently inhumane. The humanity is in the hands of the operator. In my practice, an effort is made to perform most procedures without a twitch. However, used properly, it can sometimes eliminate the need for sedation.
To safely twitch a horse using a rope twitch, start with a haltered horse. Before you even pick up the twitch, grasp your horse’s upper lip firmly to test their response and, hopefully, desensitize this area before you apply the twitch.
If your horse withdrawals violently from your grasp or doesn’t allow you to grasp their lip after several tries, you may not be able to safely apply a twitch to your horse. It may be wise to stop at this point and seek help.
Left (near) side application. Halter your horse. Place your right hand on the side of the halter, steadying your horse’s head in a "frame" without excessive pressure. Thread the fingers of your left hand through the loop of the rope twitch, keeping your pinky finger behind the loop. Grasp your horse’s upper lip with the left hand 3 middle fingers and thumb. Now using the right hand, slip the loop over the left hand and slide it well onto the lip. Now twist the handle toward your horse’s eye, tightening the loop around the lip until it is secure.
If the horse continues to struggle, tighten the loop more. As the horse relaxes, you may release slightly but not so much that the loop comes loose.
You can throw the lead rope over your horse’s neck or hold it in hand, but you should hold the twitch with both hands and stand on your horse’s side. Do not stand in front of the horse, because some twitched horses will attempt to strike with their forelegs. If the horse struggles violently, rapidly untwist the rope.
Tips for safety & Success
It takes a moment for the endorphin release, so wait until you see your horse relax before performing whatever procedure you plan to perform.
If you do not understand "feel", it is dangerous to use a twitch. It is harder to over-tighten a rope twitch than a chain twitch, but it can be done.