Benefits and Uses
The cat/cow stretches are perfect for warming up the body and creating length and mobility in the spine. The poses can be used on their own to alleviate back strain or pain, as well as in preparation for a yoga posture (asana) practice.
Directions 
Begin in a table top (all fours) position on your mat with a flat back, aligning your wrists so that they are directly beneath your shoulders, and your knees directly beneath your hips. Alignment is important to prevent injury and create maximum support by the natural stacking of your bones. 
Linking the breath to your movements, inhale to arch your back, reaching your tailbone to the sky and chest forward (cow). Exhale to round your spine - your head and tailbone reach for the earth as your middle back reaches for the sky (cat). Let your head and neck follow the movements of your spine. In the cow pose, avoid crunching your neck by looking upward; instead gaze softly forward, keeping your neck long.
- Breath: Link your ujjayi breath to each movement of the spine. With an inhalation reach your chest and tailbone forward (arching your back); with an exhalation, round your spine, expelling all of the air with the slight squeezing of your belly.
- Modifications: It is common to feel wrist or knee discomfort in the tabletop position. Create a small layer of cushion to place under your wrists; your palms and fingers will be off of the padding and this will create a small angle declining from your wrists to fingers (in order to increase the angle between the wrists and forearms to avoid crunching). In addition, visualize that you are evenly distributing weight throughout your whole hand instead of just your wrist, as this will also relieve pressure.
For knee sensitivity, place extra padding under your knees until you no longer feel sensation. Note that it is also an option to create these spinal movements from an easy-seated position if tabletop is not an option for your body. - Release: After you have taken several rounds of these movements, return to a neutral spine (flat back) and come to sit or continue your practice with another posture or warm-up series.