The less in control Zum is, the more important using just one rein is. I am going to bring one rein towards the saddle with enough tension that Zum feels contact. When Zum moves his head in the direction of the rein, I release the rein. Zum feels rewarded. I do it again. I hold the rein locked against the saddle until Zum gives me the right answer. I release the rein so he feels total reward.