This is a great basic movement to get a feeling for what is meant by "hollow" and what is meant by arched. Having facility with pelvic rocking is essential to good control of the core and torso for all movement - from squats and deadlifts to push ups and leg lifts. It is also great for loosening up tight lumbar erectors.
Work on increasing the height of your pull ups up to your chest and increasing the depth of your dips, getting your armpits down to the height of your hands. Key is also in the "false grip".
Different types: walking, forward, backward, lateral, multi-directional
Try adding weight: easiest at your sides, difficult over head