This is one of the best postures for strengthening and toning the abdominal muscles and organs and working the bottom of the back. You'll find Navasana very difficult at first, but do it regularly and you'll soon see the difference.
How to do it.
Start sitting on the floor. Lean back gently and flex your knees. Holding the bottom of your thighs with your hands, straighten your lower back. Clasp the palms of your hands together and unbend your legs until they're completely straight. Your feet should be higher than your head, and your should feel your abs working hard. Be careful not to fall backwards and keep your legs straight.