Many times I am surprised that there are people who don't do ground work. They complain that their horse is a jerk on the ground, but when someone suggests doing ground work, they look at them in confusion or say "Why would I do that?" There are many reasons for doing ground work, including but not limited to teaching your horse manners, freshening up your training schedule, making your horse a dream to handle on the ground, preparing for doing things under saddle, and during times when you can't ride, ground work gives you something to do.
What is ground work exactly? Well, it's teaching your horse things like how to walk an appropriate distance from you, how to stop when you stop, turn when you turn, etc. Teaching them to step over when you need them to move, handle feet, ears, etc easily, introduce new objects. It's also lunging and long lining (or ground driving), and generally teaching them things that make it easier for you to handle them on the ground.
One thing I see a lot is horses who don't know how to lead. What I mean is that they don't stop when you ask them to, drag you around when they feel like it, won't back up or move out of your way and require a lot of strength to handle in general. Most of these horses end up with a stud chain. Why? Mostly I think its because I think the horses are either afraid of the horse or they see everyone else doing and think why not?
There are several different methods for applying a stud chain and while they can be used humanely (they are always advertised as "humane") most of the time they aren't, or are being used as a substitute for proper training.
Under the chin. Common for 'correcting' the
horse, but little other value.
Over the nose. Used in unruly horses who won't
stand still, run away with the handler or rear.
A lip chain acts against the gums and has very severe action.
Tell me, does this horse look happy?
I can understand using a stud chain if you are an experienced handler who has a horse that has a lot of explosive energy some of the time and you can't control them with proper training or you are in a new situation and you have an excitable and naughty 1200lb horse charging across the arena. They can also be used in vet procedures if your horse decides, for instance, they would not like their eye looked at, but again training is a huge factor. Stallions are the most common recipient of the stud chain, thus its name, because a lot of people are afraid of them and these horses aren't trained to behave properly regardless of their hormones.
While the stud chain can be a tool to use humanely, most the time it isn't. Most of the time I see horses being jerked around with a stud chain that gives no release, it is yanked on very hard repeatedly for no good reason, the horse is trying to trot in a showmanship class but when it doesn't go fast enough it is pulled on, making the stud chain go into effect, and it is used as a band-aid, not a cure for poor training.
You shouldn't have to use a stud chain to control your horse. I use rope halters that give you a little more leverage than a webbed halter, but are still nice and soft with a generous release and no where near as severe as a stud chain. Horses are less likely to lean on a rope halter, but they won't be hurt by it. You can use just a tiny amount of pressure on a rope halter to communicate with your horse or a lot if they're being bad and you won't make your horse rear over backwards. When I mention maybe using a rope halter so that their horse will stop running away with them, people often won't consider it because rope halters are "hard on horses" when in reality they are WAY more humane than doing anything else. Your horse will thank you!
If my horse wants to take off, I can control them with a plain halter no stud chain needed. They know that steady pressure means to slow down or back up. I don't have to put a lot of muscle into it. If they're not paying attention to me then a sharp tug gets their attention and we're good. If you put some work into teaching your horse to lead properly then you'll get that same amount back with regards to your horse learning how to lead. With some people it just comes down to laziness. Why teach your horse to stay out of your personal space when you could be riding?
Jetta came from a farm where stud chains were used on all the horses as a matter of principle, so she came to me with no respect for my personal space and unless you used a stud chain, pressure meant nothing to her. I think it only took two or three sessions working with her until she understood how to stand still and move with me, how to stop from a trot and keep her focus on me, not the other horses gallavanting around the pasture, and that she should respond immediately to a small amount of pressure.
Overall, stud chains do have a (very, very) small limited amount of circumstances in which using them are appropriate. Otherwise though, training can result in a horse that doesn't need one despite the situation. Regardless, everyone has their different views. Here for pro stud chains are Eventing Nation (look for the Shank You Very Much article), I think she makes some very valid points and Kristine Oakhurst, who I don't agree with at all. But there you go. If you need a lot more leverage, an alternative is the Humane Stud Chain.