Start: Get down on your hands and knees. Keeping your back flat.
Finish: Take a deep breath, allowing your belly to pooch out. Then forcibly exhale and round your back like an angry cat as you lift your navel up toward your spine. When you can exhale no more, keep your back rounded and your navel in as you purse your lips and take shallow breaths through your nose for several seconds. That's 1 repetition; it should take 20 to 30 seconds. Inhale as you flatten your back to the starting position.