In your two-point you are pivoting on your knees. What I mean by that is that your body is like a board teetering with the fulcrum being your knee. This could mean one of two things- you are stiff or your leg is not strong. I don't know what kind of shape you're in, but I'm guessing you're stiff. Your hips should be following your horse's canter and moving with the stride. The motion is similar to a hula hoop type rolling movement. This is not a movement in your back, merely a slight movement in your hips. The best clinician for that problem that I've seen is Daniel Stewart- plus I love his energy and sports psychology seminars :) I hope that helps.