Slowly bring them down and, just as slowly, allow the torso to come up to the original position. Rest for 10 seconds. Once again, slowly lean back and repeat the movement. The number of sets performed are up to you. It would be fair to say that after 10 minutes of this type of training, your abs will be screaming for mercy.
Abs are stubborn.
Fat loves to hang around the waistline. Perform this routine on days when you aren't working any other bodypart. Hit them hard as you would legs or chest. After a couple of weeks, you may start seeing definition in your abdominal region you never thought possible. Once you've achieved more muscularity and less fat, it will be easier to maintain.