This type of board?
Keeping your knees bent, you should start with your back straight and on the board. Lower the board to the lowest position. If it's too hard in this position raise the head of the board by setting it on something solid, like a chair. As you get stronger, gradually lower the head of the board until it's on the floor, then finally start raising the board at the feet end.http://exrx.net/WeightExercises/RectusA ... SitUp.html
"Movement can be made easier by lowering incline, performing movement on horizontal surface, or placing arms further down away from head."