Imagine letting your eyes lead the movement, looking to the left to see something on your left side, and allowing your neck and your shoulders and your torso to respond fluidly to that movement. Sit quietly and register your experience. Then actually turn your head from left to right. Notice how far you go on each side. Notice the quality of the movement.
Some people even find that the left side feels smoother and more fluid than the right.
It had to come from something else, from the learning and change in awareness produced by imagining the movement. If you didn't experience change from this imaginary movement, don't despair. The imagination is like any other muscle. It gets better the more you use it. Here's another way of experiencing the difference in awareness that selective attention can create.