Instructions
Sit on the floor with your knees bent and tucked in towards your chest. Keeping your core braced, lean back to an approximate 45 degree angle and raise your feet off the floor. With your hands on the floor by your sides, extend your legs out in front of you and lean your torso back to counter balance. Return by tucking legs back in towards your chest by bending at your knees and hips and pulling your torso forward to maintain balance. Repeat.
Tips
To increase the intensity of this exercise, keep your hands off the ground and have your arms out in front of you, with your fingers touching your feet when you tuck your legs into your chest.