How to do it: Lie faceup with your knees bent and turned out, feet flexed, and heels pressed together. Inhale and lift your head and shoulders off the floor, curling up over your ribcage, looking at your legs. Reach your arms outside of your hips off of the floor, palms facing down
As you exhale, press out through your heels and extend your legs 45 degrees, squeezing the backs of your knees together. Inhale and bend your heels back into your body. That's one rep. Do up to 3 sets of 10 reps.
Make it harder: Extend your legs lower to the ground as you press out, or keep both legs extended and turned out, and then lift and lower them instead of bending and extending (just be sure your back stays down during the movement!).
Mind your muscle tip: Focus your attention on your abs, forcing them to do most of the work to lift and lower your legs. Be sure to keep your lower back pressed into the floor the entire time.source by www.shape.com