You are working on the flat and feel your horse’s haunches shifting to the outside. Your outside leg is moving back asking your horse to shift the haunches back on the track, but he is not listening. Are you sure you the haunches out are the real problem?
Very often I see riders trying to fix a symptom rather than the cause. In the case above the haunches shifting to the outside is a symptom of the inside shoulder being stiff and the horse leaning on the inside rein instead of being soft and bending around the inside leg.
Because of biomechanics he has to shift his haunches to the outside when he is leaning on the inside shoulder and rein. If you first fix the should and get your horse to soften and bend properly around that inside leg, his haunches will automatically straighten out, and his hip will be perpendicular to the track again, following the shoulder.
Horses do not like being off balance, just like we do not like it. However if you could lean onto someone or something that would hold you there would not be any need for you to seek balance on your own two feet. The same goes for the horses. Most riders when they feel the horse leaning on the inside rein they tend to pull on it which just makes it easier for the horse to lean on it more and he does not have to rebalance his weight over his own four legs. This will actually teach him to become heavier and lean more rather than be soft, light and in self-carriage.
Only asking him to move away from the inside leg and shifting his weight back onto the outside shoulder, and then softening when he does it will truly teach him to be balanced and soft. As a result of that the haunches will no longer be shifted to the outside because the horse is truly bent through his ribcage. Bit, shoulder and haunches are perpendicular to the track and the horse is balanced within himself and in relationship to the track you are asking him to be on.
If you always focus on this, you will automatically go to inside leg instead of hand to help your horse balance. This will always help you with flying changes; better roll back turns in the Jumper ring and a more handy horse in the Hunters.
As always, practice doesn’t make perfect only perfect practice does, and your horse does not know the difference, he will only repeat what you teach him, right or wrong, good or bad!! Make it count teach yourself and him good habits instead of bad ones, they both take the same time to learn, however undoing a bad one takes a lot longer than teaching a good one.