The abdominal crunch machine is one of the most misused (and probably unnecessary) pieces of the equipment in the gym. You sit on this machine and push against a pad as you crunch forward against resistance. It is inadequate because there are more effective ways to work the abs, and the value of crunches, is questionable (see posts from Dec. 11, 2011 and Nov. 7, 2010).
This machine is misused in a variety of ways: Many people do not adjust it correctly. This can put stress on your back. If you insist on using this machine, make sure you center the axis at your waist. Adjust the weight correctly: it should be heavy enough that you can do 12-16 reps without momentum, but not so light that you can do 100. Make sure to move from your torso, not your head, neck or arms. I see people working their arms and neck more than their abs. Better yet, do crunches on a stability ball, or chops with a medicine ball.
(Hold a weighted medicine ball at your side, pivot from the hip and reach the ball above the opposite shoulder). You will get better results, and with the chops, an exercise that can have carry over in your day to day life.