These two poses are usually taken together in coordinated movement with the breath. The idea is simply to begin warming up the spine, gently loosening the structures of the spine as well as increasing circulation to the vertebral discs. You will find these movements in most classes and for good reason. It’s not only an excellent way to warm up the body, but a great way to develop an awareness of the unique state of your body before moving into more strenuous postures.
Begin by coming to tabletop (all fours) with the hands right under the shoulders and the knees right under the hips. Taking an inhale, tilt the pelvis forward as you lift the the chest and the gaze gently upward. Keep the shoulders drawn down the back so that the neck stays long. On the exhale, tilt your pelvis back, lifting your spine toward the ceiling while tilting your head forward to look at your bellybutton. Keep the movement smooth and controlled and try to feel the subtleties of the spine.
Feel free to find other ways to move the spine here: you don’t need to only move straight forward and back. You can look side to side, move the spine in a circle; really anything that helps you to loosen the spine and become more aware of your central axis.