To increase flexibility and stability of the lower back by flexing and extending your spine. Also increases flexibility to the thoracic and cervical spine.
Position yourself on all fours with palms flat against the floor beneath the shoulders, and with thighs vertical beneath the hips. Start with your spine in the flat / neutral position.
Take a deep breath and contract the abdominal muscles and hold the breath. Now arch your back upwards, like a cat or bridge, by contracting the back and abdominal muscles. In the arched position, stop, fully exhale and increase your abdominal contractions. Hold this position for 5 seconds. Return to starting position and inhale.
This may feel like an alien movement to begin with, try using simultaneous straightening of the elbows and protraction of the shoulder blades to aid with the arching of the lower back. If you have a spinal problem and for more advice consult your osteopath before doing this exercise.