On the side of those who think that "have to" is not part of the Clicker Training process, I agree. One of my basic premises is that the horse has the choice of whether or not to respond to any of my cues (which differentiates a cue from a command in my opinion). If I give a cue and the horse does not respond, that is information, not defiance. It could very well be that I have not trained it well enough, or have not trained it well enough to generalize to all situations (the difference between a horse who goes wonderfully at home but things fall apart in a new situation). It could be that my cue was unclear in a given situation, or that I was not paying attention to see that the horse was distracted. If I am in the mindset that the horse "has to" respond, then I am closing the door to all that information and my training will suffer as a result- I have eliminated the possibility of feedback from my horse until, well, it's a dire situation and the horse feels he needs to react in a very strong show of emotion to be heard.
On the other side of the question is the people who say that there are times when a horse has to respond in certain situations. Who wants to ride a horse that decides not to stop when asked? Or even lead one for that matter? It's dangerous.
So I think it's important to define what we mean by "have to" and what training methods we're going to use to get there. For many trainers, "you have to" means there will be consequences if you don't. In fact, there are always consequences- we don't live or train in a vacuum. So what will those consequences be? Do we ask again? Ask in a different way? Make a training plan to address this? Those are all what I would call clicker compatible consequences to a horse ignoring a cue. Others may give up. This is what many beginners do because they have no tools with which to address a problem. Still others will escalate- kick harder, pull more, put on stronger equipment. And others will punish- a whack with a stick is just one item in a collection of punishing responses. Those are not clicker compatible consequences.
Another question to ask is what behaviors you think a horse "has to" respond to and why? Is it because you don't want to be embarrassed in front of other people or at a competition? Is it to keep you comfortable? As in, "I don't really feel comfortable with this horse trotting through here and I'd rather be walking". Or is it safety? "You HAVE TO stop right now or that truck is going to hit us both?" Certainly the last is a reason I would agree with is a situation that I would say the horse HAS TO stop. And feeling comfortable is very close to that same need. Feeling embarrassed? Well, as clicker trainers, we're pretty used to people looking at us like we've lost our marbles anyway so this is just more practice for letting that roll off our backs.
So how do we convey "have to" using Clicker Training? We train. We make "have to" equivalent to "I'd be happy to". We Think, Plan, Do, to quote Susan Garrett. Think about what we want (as opposed to what we don't want). For instance we want a horse to stop when we ask. We come up with a plan, using all the tools that Alexandra Kurland and other clicker trainers have generously shared. And then we train: foundation lessons, baby steps, regular progression, assessing progress, plan some more and train some more, reinforcing every step of the way so that the horse says "stop? I'd love to" and he stops.
I think "you have to" implies some sort of threat out of the fear that the horse might refuse. But if the horse learns to love to work with us, why would she refuse? Well, shit happens, as they say. Things don't always go according to plan. A situation may arise that we have not had time to train for. Or we may not have thought to train for. We may have been building up a great response from our horse to our stop cue and may have a horse who walks politely along with us, following our cues for stop and go without a problem. But then early one morning a wild animal goes through your pasture fence, tearing it down and scaring your horse silly so that she gets out. You may no longer be in your safe training area and may not have trained for the neighbor's property yet. Plus you have this horse who's acting half out of her mind thanks to the intruder. Now what?
I think the answer is different for all of us, depending on our own personal emotional state at that point. I know the more I have clicker trained, the more confident I am in those situations. A chain over the nose never made me feel as safe as having a horse who knows how to calm down on cue. Even though, in a hairy situation, she may not go into complete responsive relaxation, the training that has been laid down gives me the tools to ask her to relax....traditional training never gave me that. It gave me tools to be bigger and tougher and threaten good behavior, but a simple touch on the poll which simply means relax? Magic. If I get even an inkling that I have been heard in that request, I know I'm on my way. The trust that builds up over those training sessions- putting a horse in a mildly uneasy situation and helping her learn emotional control- will mean that she is more likely to look to me for assistance rather than feeling threatened by my "you have to".
Think of yourself in a tense situation- if there is someone there whom you trust, doesn't that in itself make you more comfortable? And if that person suggested a way for you get out of that tense situation, wouldn't you be likely to take that person's advice?