What You'll Need
An ab roller (think a wheel with two handles), and something cushioned for your knees to rest on. Heaven forbid we start mistreating those precious knees of yours. Oh and if you feel like making it a little more difficult, do it on a bosu ball (those half exercise balls with the blue tops).
Why You Should Be Doing This Exercise
This may, in fact, be the best core exercise you can do in terms of working the entire abdominal chain. Try it and realize the resulting soreness reveals its importance.
Get down on your knees and place a hand on each of the roller handles. At this point, you should be on all fours, with your knees either on the bosu ball or on the padded surface.
The goal here is to roll the ab roller forward with your body following (knees remaining on the ground) until your body is as close to the ground (without touching it) as possible. Your arms should be completely extended in front of you.
Once your body is hovering just above the ground, hold it just for the sake of inducing pain, breathe out, and then pull yourself back up. The pull is interesting because your triceps and arms will volunteer to do the work, but your abs, as Captain of this ship, should steer it home.
Focus on your abs stretching out as you roll out. Don't fight that from happening. Also, don't collapse in a defeated heap when you're fully extended, because the goal is to not touch the floor. But you wouldn't think of doing that now would you?