Many yoga classes include some cat/cow motions, usually near the beginning of the sequence in order to warm up the spine. Have you ever wondered why we do them? Is it just so we can stretch out our backs? Is it good for breathing in and out? It all made a bit more sense to me once I started really connecting my breath to the movements.
The goal of this exercise is to bring your spine into full extension and then into full flexion, not only globally, but at each segmental level. Starting from a neutral spine in all fours (tabletop) position, inhale, look up, tilt your head up, and then find extension of your occiput on your cervical spine, then C1 on C2, C2 on C3, and so on through each segment of your cervical, thoracic, and finally lumbar spine. Think of it like a wave starting at one end of the spine and propagating clear to the other end before changing directions (as you begin spinal flexion). Use your breath to awaken the areas where it feels difficult to move one vertebrae with respect to it’s adjacent. You can start the “wave” either from the top of your spine or from your tailbone, just as long as you focus on moving a small area at a time. You will feel more benefit from the exercise if you do it this way, than if you think of it simply as “lifting up, lowering down, lifting up, lowering down”.