The Butterfly Situp is a fantastic ab exercise because the manner in which you position your legs forces your abdominal muscles to do more work since your hip flexors become disengaged.
1-Lay on the floor, face up. Bring the bottoms of your feet together, your knees pointing in opposite directions.
2-Using the power of your abdominal muscles, bring your torso upright, eventually rounding over your legs and touching your toes with your hands. Exhale throughout this motion.
3-On your next exhalation, slowly lower your torso, vertebrae by vertebrae, back down to the ground. Again, this is done with the power and control of your abdominal muscles. This is one repetition.
Do as many reps as you can in 1 minute.
It is important that you do not throw yourself up and down and are certain that your abs do the work. It can hurt your back and/or completely avoid working your abs if you do not control your core.