A stallion chain works in the way a rope halter does.. but perhaps a bit more firmly.. to put it politely. Its in no way cruel if used correctly, as with a rope halter. Its a chain that attaches to a headcollar and goes across a nose, and attached to a leadrope- depending on the hands depends on the results. If you're nervous around your horse, and your horse takes advantage it may take more than one use, and sometimes continuous use. I don't agree with this. If you have more capable hands, you should only have to use it once. Maybe twice at a push, I did when I first bought my mare- 18.1hh is a lot to hang on to when it says NO. Once it was used, and when I do groundwork, a knotted halter. But then you get, with everything, the 'cowboys' and I don't actually mean cowboys, just a term, that see fit to yank and pull and put as much pressure as possible on a horse's nose.
By classically, I meant traditionally. You teach the horse, the horse understands its job. If the horse is always wanting top position, as you get with some, greenies, stallions, mares, some bloodlines, and the horse tries you, you ask nicely once, and then correct by any means necessary.