As I was mucking stalls this morning, I was thinking about horses in my past and the many horses you see in the stabling area of competitions who are rock steady in their manners on the ground. They will stand while they are groomed, wrapped, braided, etc, frequently without even being tied. But for many of these horses, it's because they have learned it's better to do nothing until told to do something. So they stand still unless they are pulled (rope) or pushed (shoved over). Rather than being taught TO stand, they have been taught NOT to move. There is a difference.
The first difference is the way that it is taught- with reinforcement or with punishment. Clicker training horses to stand still is done with reinforcement- stand still and you get a treat, now stand still longer and get another treat, etc. They learn self control leads to rewards. They learn good things happen if they stand still. Horses who are taught NOT to move are usually taught with punishment. If they move, they get yelled at, slapped, shanked, yelled at some more, etc. Pretty soon they are afraid to move because if they do, something bad will happen...that's fear.
The second difference is what will make these horses move and what happens when they do move for some reason. The horse who has been taught not to move by using punishment will stand until he becomes more afraid of something else than he is of the punishment. It may be a door banging, a dog barking down the aisle, a loose horse outside, kids running and screaming etc. The same things might cause a positively reinforced horse to startle but it is more likely that his frame of mind is calmer. And if he does startle, the response will be different. Rather than over-riding fear, he has been conditioned to think about the good things that will happen if he stands. So rather than spooking and then taking off for fear of the retribution, he will spook and then think, "oops, if I stand or go back to my person, good things will happen". A horse standing out of fear may be on edge on a windy day because he is nervous but is afraid to move for fear of punishment so the tension builds and builds in his body until he busts out with some nervous energy or becomes so afraid that he decides it's better to leave town than stay put. A horse standing from being rewarded will have a calmer, happier frame of mind and will be conditioned to calm himself in order to get rewarded. In addition, his person will most likely see his nervousness and begin reinforcing him more for standing as well as asking him to do relaxing things like dropping his head which is a conditioned activity for calming. That horse will feel safest with his person, doing what he knows will be reinforced. The other horse may very well decide that he needs to take matters into his own hands since his handler is scary and so is the banging door.
In the end, I'd rather have the reinforced horse under me when riding than the punished horse. I feel safer on a horse who looks to me for help in calming down than the one who may decide that truck coming down the road is scarier than the thought of any punishment I would dole out, especially if he dumps me just to avoid both options.