A leaning horse is a lame horse. It's only a matter of time. Horses are not bicycles or motorcycles, there should be no leaning in turns. Our horses should shift their weight to the hind legs and swing their trunk to the outside of the turn while bringing their shoulders through the turn. All the while not overflexing in the neck which throws the weight in the shoulder (outside shoulder if rider is pulling the inside rein. Inside shoulder if the horse is telescoping the head to the outside of the turn). The over flexion of the neck also creates bracing in the poll as the horse works to stabilize itself. The tops of the spinal processes should stand up facing the sky not leaning and pointing towards the horizon. When leaning is occurring there is dysfunction in the dynamic movement of the horse. This dysfunction will stress joints and recruit muscles to stabilize areas that have become unstable due to the lean. Over a long enough period of time, the body breaks down. This leads to behavioral problems from pain. A painful horse is a lame horse. Slow methodical work can stop leaning. You have to align the spine, keep the base of the neck stable and teach the horse to rotate the trunk both directions. Most of this happens from your seat and leg. Your hand will exacerbate the problem if used as the primary aid. Preventing lameness always takes less time than rehabbing lameness.