I know that some people have trouble working out how you can use clicker training to make the horse more forward, when the result of the click is always a halt (to get the treat).

I don’t think that Charles De Kunffy is a clicker trainer, but he has answered that question in his book ‘Dressage Questions Answered’. He says;

“Where do riders go wrong? ….they go wrong by trying to use as means their goals! So they get a different end result because the one they wanted was spent as a means and those means create an unpredictable result!

So remember that a rider who pursues his goals by riding his goals will never achieve them. But a rider who pursues various proper means leading to a goal that is a genuinely new condition, will progress.

It is well for us to remember Hegel’s dialectics that those who confuse the means with the end are the losers.”

He also says, regarding straightness and impulsion;

“Remember that when we define our goal as being a horse that moves straight forward with good impulsion, we employ the means of bending, for it is through bending that the horse becomes straight. That is why riders who just ride straight on their green horses will always have green horses, because through straightness you get crookedness but through bending you get straightness.”

He also says that there can be "no balance without relaxation" and;

"Relaxation is the real name of all stages. For whether it is the rhythm or the engagement or anything else you might work on, it is only a sophistication over relaxation. Engagement is committed relaxation….. The horse’s musculature and joints are supple and elastic because they are relaxed. That is why in the ‘pool of relaxation’ balance, rhythm, impulsion and engagement, which are all developmental sophistications of relaxation, must be cultivated.

Any time spent on a tense horse is a waste of time."