There are a lot of different twitches out there. Twitches take the horse's attention off whatever is bothering him (clipping his ears, pulling a tick out of the ears, medicating, vetting, etc.). Some people just twist a horse's ear and hang on to twitch the horse, but it's hard to hang on that way, it's not always effective (it's hard to maintain that position for long) and it can make a horse head shy (No one is putting their hands near my head again! Good luck bridling or doing anything else near my head. . .). I don't recommend that type of handling unless you have nothing else to work with and it's an emergency.
Twitches, while they may look 'mean', don't actually hurt the horse. Quite the contrary---with a twitch on, a horse is distracted enough for you to safely do a task (like clipping) or vet him without the horse bouncing around and putting everyone in a precarious situation. If someone grabbed your nose and pinched at the same time that you got a shot, do you think you'd actually feel the shot? Some horses will sit there and move their upper lip around while the twitch is on (like in a curious way, not in a panicked or get-this-off-of-me way). Others will just stand there and let you do whatever you have to do without dancing all around and stepping on toes and such.
One type of twitch is simply a loop of chain on the end of a short wooden handle; you put the horse's upper lip in the chain and twist until it is tight. The problem with that sort of twitch is that someone has to hold onto it at all times, and usually that person is directly in front of the horse---if the horse kicks or rears, that person is in harm's way. And if they don't hold on tightly enough, the twitch can come off, possibly putting the other person in a dangerous situation (like if they are palpating and suddenly the horse kicks or something).
The humane twitch can be used by one person. You put the horse's lip between the straight parts of the handles, squeeze tightly, and then wrap the string at the bottom of the handles around both of the handles (as tightly as you can) and clip the snap at the end of the string to the horse's halter. Now you have a twitch on the horse, no one needs to hold it, and it is a lot safer for everyone. Some people try to put the upper lip through the bent part of the twitch (near the top), but it's bent like that to prevent a horse's upper lip from being 'nutcracker-ed' and seriously pinched.
The only problem with this twitch is that if the horse moves his lip around a lot, it can work loose and fall off, and then you have a potentially 'scary' object hooked to the horse (try getting anything off a panicked horse without getting hurt yourself). But that doesn't happen too often with most horses. You're more likely to get the handles slapped in your face if you forget the horse has a twitch on and the horse's nose is near you when he shakes or turns his head.
The other problem with this (and any twitch) is that if you use it often enough, the horse won't let you put it anywhere near his lip. It's really hard to get the twitch put on a horse who is 'smart' about what is about to happen. They can lift their heads high and make it a real job to get the twitch on. But once you get it on, it usually works. Most horses respond to this. In over 20 years of dealing with thousands of different horses, I can remember only a couple whom the twitch wasn't effective on.
I'm sure it's not the most pleasant activity for a horse, but it's a lot better than not being able to give a horse medical attention, or watching someone beat a horse because it won't stand still for something like clipping. The average horse weighs about 800-1000 pounds. The humane twitch is a tool to help you care for them safely when they are scared or in pain.
I've used a lot of these twitches at various barns and vet clinics, and I've never had one fail. Even after many uses a day (in vet clinics) on many different horses, they are in perfect shape. They are the "Tonka" of twitches. They are built to last. It's really cheap insurance. . .
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