Summarize this article in one sentence.
Place the ab rocker on the floor with the headrest behind you. Lie down on your back inside the rocker so that your head is resting on the headrest and the upper roll bar is positioned above your chest. Make sure your head, neck, and back are straight, and that your back is resting flat on the floor. Once you're in position, bend your knees and put your feet together, with the soles flat on the floor. Raise your arms and lightly grip the upper roll bar above your chest. If you're a beginner, place your hands next to each other on the roll bar, directly above the middle of your chest. For more difficult crunches, place your hands shoulder-width apart. Slowly exhale through your mouth as you use the ab rocker to help you rock forward, lifting your head and upper body off the floor toward your knees. Make sure your abs are tight as you perform this action. Take care not to push too hard with your hands as you perform the crunch. You still want your abs to be doing most of the work. Use slow, controlled movements and make sure your head does not slip off the headrest. Once you've performed the crunch, hold it for 1-2 seconds before returning to your starting position. Inhale as you slowly lower yourself back down to the floor. When you're starting out, you may wish to do 3 sets of 7-8 crunches. You can gradually increase your number of reps as you get better at doing crunches.
Lie down with your head on the headrest. Bend your knees and place your hands on the roll bar. Exhale and bend forward with your abs contracted. Inhale and return to your starting position.