HOW: Start by laying flat on your back. Perform the exercise by flattening your low back into the ground, then lift both legs up straight towards the ceiling. While keeping one leg still, slowly lower one leg down towards the ground as low as you can control, then lift it back up and switch. Repeat.
FEEL: You should feel your abs, legs, and core working with this exercise.
COMPENSATION: As you lower one leg down, try to keep the other one still. Do not let your low back excessively arch. Try to keep your low back pushed into the ground. Do not let your knees bend, keep your quads squeezed to keep your knees straight.