s are for wussies
es are for limp-wristed
pansies. If you want to develop abdominal muscles, try the Angle balance.
It is advisable to perform the Yoga warmup before this exercise to prevent overtightening the back and neck muscles while performing. You need plenty of space, and a low-friction surface to perform on when swiveling.
Start sitting up, legs together stretched out in front, with your hands behind you and your fingers pointed forward, toward your feet.
Position by inhaling, picking up your legs as far as you can so that they are locked, and you can balance on your rear with a little support from your hands. Hold your breath like this for 10 (one-one-thousand, etc) seconds. Exhale, and lower your legs down so that your feet do not touch the ground, but they are still close. Make sure your knees stay locked when you go down. Now, keeping your feet above the floor, swivel 90 degrees clockwise, and raise your legs again keeping your knees locked for 10 seconds, as before. Repeat it until you face forward again, or 4 turns.
To leave the pose, come down, stretch forward and touch your toes. Suck in your abs to stretch your back. This is absolutely necessary. Repeat as many times as necessary.