Most people use momentum and jerk their body up and down by kicking their legs to create the movement. Eliminating that momentum forces the abs to perform the action.

Once you get your legs up over your body, slowly reverse this motion, lowering your spine back down toward the bench, one vertebra at a time. Don't allow your legs to extend or your head to lift off the ball at any point.

Related:  Big, Thick, Chunky Ab Training

Related:  Dynamic Training for Abs & Obliques