Instead of moving his haunches and body, he barreled through my right rein with his shoulder, and gave me a huge middle finger. I jerked him to halt and then picked up the right lead canter again. And again he blew through my rein and again I halted him. We ran through the exercise until he finally started to respect my right rein, and suddenly, he could move laterally. He gave a very good flying change, and that was it.
The next time I rode him, there was no fight in the exercise, and he did it correctly in both directions. The flying changes were smooth and easy. This exercise is now my go-to for fixing a dragging hind end. Here's a quick video of riding it with Chemaine explaining.