They work through the extended and collected walk with the poll in the correct position for the movement.  In the canter he helps the rider with the correct rein length and hand position, reminder her to allow her elbows to move in a circle.  In the canter they work on the medium and collected and half pass, adding a flying change.  By having her work on her body position he improves his canter. They ride shoulder fore and travers, making sure he moves away from owner left rein.  When they add the half pass they also do the single changes, and Nicholas points out that the problem is not the change, It's having the right canter for the change.