Source: Creative Commons, Wikivisual
4. The floor crack: Lay down on your spine on hard flooring. Keep your arms stretched out and your knees bent, feet flat on the floor. Your lower spine should be against the floor. Interlock your fingers at the back of your head and push your head forward till your chin reaches your chest. Press firmly into your chest. Note that this posture should not hurt, if it does, apply less pressure.