Lying on a mat with your legs up in a slightly bent position, bring your arms straight out at shoulder level. Keeping you shoulders flat to the mat with a pinch in your shoulder blades, let you legs fall as far as you can to the floor while keeping you back and shoulders on the mat. Flip the palm up to the side the legs are falling to, and switch as the legs cross the mid line of the body. Breathe out as the legs get towards the ground.
Why it works
This is a combo of core, posture and flexibility. If you have excellent flexibility, straighten your legs.