Imagine your right arm is a string pulled taut from handlebar grip to right shoulder. From the grip that string goes rearward and upward. Now imagine you are pivoting your shoulders and torso to the left, as most of us do when looking over our left shoulder. Your right shoulder also moves to the left. Now the string has to go rearward, and upward, and leftward. It can't stretch, so it can't do that - to add the leftward reach, the string has to go less far rearward or less far upward. Dipping your right shoulder reduces the upward reach and lefts the string reach leftward.
That makes sense. Hopefully I'll get good at that movement soon!