HOW: Begin by lying flat on your back with your knees bent and feet supported on the ground. To perform the exercise, flatten your low back into the ground, then while maintaining this position lift your legs into the air so that your knees are stacked over your hips with your knees bent at a right angle. Hold this position.
FEEL: This should like a lower ab workout exercise, not strain on your low back.
COMPENSATION: Keep your low back pushed into the ground, do not excessively round or arch your low back.