Begin by lying flat on your back, with your legs straight up at a 90* angle to your body and your toes pointed toward the ceiling. Keep your hips flat on the floor, and stretch both arms up toward your toes. Remember to keep your legs from dropping over your abs, as they need to be straight in order to properly work the muscles.
This exercise is similar to a regular crunch, but involves a straight up motion rather than a curling or crunching motion. The higher you stretch toward your toes, the more your ab muscles have to work. When you first begin, aim to reach just above your knee, increasing your range of motion as you progress with your fitness goals.
Do not release your abdominal muscles when you lean back, but keep your head slightly elevated as you do each repetition.