I agree with Spirithorse. When we are with horses, we are the herd leader and they look to us to gauge our reaction to things.
Wait, didn't you say that horses see us as predators, and now you're saying we're the herd leaders. Furthermore, if they see as the herd leader, shouldn't we ACT like the herd leader? If a lower horse is disrespectful, they'll get a warning, then they get nailed.
If we hear a whip crack and we jump, 99% of the time, the horse will jump too. If we are nervous, then they think that maybe we are not a good leader and will take over as the leader. This is sometimes seen as bad manners, but is usually because the horse doesn't trust our leadership abilities.
Australian NH trainer Carlos Tabernaberri says that 'we should not punish the horse for reacting to our reactions'. For example, the owner is nervous and jumpy on a windy show day. A bag flies past and the horse, seeing that the owner is nervous, assumes they aren't a good leader and tries to bolt. The owner sees this as being naughty, flighty or silly. Two things can happen. The owner can hit the horse. The horse will probably stand still again, but the next time a bag flies past they will have the same response. Or the owner can prove to the horse that the bag is not going to hurt. This also comes with a relaxed owner. Next time the bag flies past, the horse will understand that the bag won't hurt them.
I agree, if we're nervous they'll be nervous. Personally, I have never met a good horse person who would punish a horse for being startled by something, especially if we jumped as well. The best way to deal with it, IMO, is get the horse busy. Give them something to think about, beyond the scary plastic bag. Old horse I used to ride used to practically look for things to spook at when he was bored. But if I kept his brain busy by keeping his feet moving, he was fine. "Oooh look, a bag. Wait, I have to leg yield now. OOH, you want me to go into a canter, okay! Wait, you want a circle here? Can Do!" Funny side note, but another horse I used to ride, someone would crack a whip, or snow would fall off the roof, and I'd jump a foot in the air (I have a bad startle reflex). He could care less, and would just keep going.
The 'founder' of NH, Pat Parelli (apparently he's big over in America), says that NH is about rejecting force as a training tool. I don't follow Parelli because it's becoming very commercial, but I do follow the basics.
Parelli sure as hell ain't the founder of NH. And just a point of interest, but I've heard from some VERY reliable sources that Parelli isn't always as 'force free' as he'd like the public to believe.
As for the ruined horse...even after we got his back fixed, he still bucked because he was expecting to get hit. Yes, it is natural instinct for the horse to be checked for pain, but no-one here, even the instructors, thought to check. Here in Australia, very little is known about chiropractors and their work. Since then I have made every effort to check for pain and to learn about it, but I was young and knew little.
I would like to add that the horse in question is going very well. Most would have sold him on, but he has potential and we kept him to help correct him. He's 15hh and has jumped 4'9!
That's good that your horse is doing well. However, I don't think 'check for pain' is a chiro thing, it's more of a general 'good horsepeople' thing. I'm not blaming you as you were young, but I do think that you may not have had the best representation of mainstream horsemanship.
I'd just like to make the point that none of us on here are horse abusers, horse beaters etc... Yes, I have had a few heafty 'conversations' between me, the horse and a dressage whip, but those were all for a specific reason. It wasn't because I lost my temper and wanted to beat the horse, it was because the horse said 'make me' and I said 'okay'. The horse knew the rules ("I will not try and kill the human"), the horse knew how to get out of that situation ("I will stop trying to run the human over when leading"), and it was done out of flat out disrespect.
Like all training, it was as little force as possible, but it was as much as necessary.
It's also important to understand what's going on inside your horse's head. You need to know when they're scared, when they're in pain, when they are confused and when they are being disrespectful. Only ONE of those situations could ever warrant flat out hitting a horse.
A horse may get a light touch with a dressage whip if he's confused about going forward, but that's not force so much as a strong poke to get their attention. A 'yes, you're supposed to go forward' reminder.