I think dipping your right shoulder allows you to use more of your vertebrae for turning your head, instead of just using your neck bones, so there's less tension on any one section of your spine, and you can maintain better balance. Dipping the right shoulder also puts your weight on the left hand, which is positioned near the center of the handlebars so the pressure doesn't torque your steering, and the arm hanging loosely from your dipped shoulder is unstressed, so it can better control the steering of the handlebar.

That's how it seems to me, anyhow...