There's an awful lots of "shoulds" and "don'ts" and "nevers" that come up whenever horse people start talking about training horses or teaching people to ride. Some of them make sense from a safety standpoint and that's important. A whole bunch of others, however, are just based on the theories and assumptions of the person talking or on their personal beliefs about what's good or bad or what's humane or inhumane. None of those things are necessarily horse logical. I'll give you an example.
Back in the late '50s when I was tagging along behind famous trainers to learn from their wisdom, I wound up riding shotgun behind one trainer's trailer on a 1942 Army surplus Harley Davidson motorcycle. Now this model was not exactly the kind of bike most people think of nowadays when you say "Harley Davidson." It was a stripped down, underpowered wimpy-engined excuse of a Harley that didn't even have a windshield. As you draw your mental picture, remember that this event also took place back in the days when nobody thought about wearing helmets. The wind blowin' through your hair and all that.
Now on account of the no windshield deal, I was wearing glasses. Which was a good thing because I was going to learn an important lesson about horses which might have been a little harder without the glasses.
The trainer guru whose trailer I was trailing had told me a lot of things about horses. He was a successful trainer so, of course, anything he said must be gospel. One of the things he told me was that horses will never void in a moving trailer. That, he said, was one of the reasons you have to pull over every once in awhile and stop so they can relax and pee. Absolutely. A lot of other horse people agreed that was gospel. There was just one problem. No one told the horse in the trailer I was following on my Harley.
The animal in question was a big 16-hand critter named Ivan. And Ivan figured that, even if you're zooming down the highway, when you've gotta go, you've gotta go. So he did. And when that yellow mist started spraying out behind the trailer, I realized I was in a bad spot. That '42 Harley wasn't fast enough to pass the trailer and dropping back just meant it took the mist a little longer to reach me.
So I learned a lot of things that day. I learned that you'd better remember to wear glasses if you're riding on a motorcycle that doesn't have a windshield. I learned that the gospel preached by successful horse trainers isn't totally infallible. Most importantly, I learned that if you're going to tell people that there's something a horse will never do or not do, be sure you tell the horse, too.
There's an awful lot of well meaning people who assume that because something worked with one horse, it's going to work with every other horse. If all of their experience has been with Quarter horse or Arabians or scared baby horses or middle aged dominant mares, they're going to be just fine as long as they keep working with that one kind of horse. They get used to doing X and have the horse do Y. Then along comes a critter that sees X differently and does an S or a Z or even a Q. Now the handler stands there and says, "Boy is this a dumb (or stubborn or lazy or something else) horse. He just doesn't get it." So they repeat whatever they were doing in a louder, more aggressive way to try to impress the horse that they're in charge.
You're not really communicating with horses if you assume a specific pressure is going to produce a specific response in every horse. To truly communicate, you need to start paying attention to how the individual horse in front of you at that moment feels a pressure as logical or not. Then you have to modify your pressure to use the horse's own logic to get your point across to him more clearly. As a way of communicating with horses, heeding works because it isn't a "one size fits all" program. It provides handlers with a set of basic communication tools but it's up to the handler to apply those tools appropriately to the individual horse.
Those individual horse response are what can make horse training frustrating. When someone e mails a question and asks me why their horse won't back or load or jump without rushing, I can't really give them a precise answer. Without actually seeing them and their horse, I can only guess what kind of logic the horse is using to respond to whatever pressures they are applying. But figuring that out is what makes communicating with horses so rewarding and so much fun.