Sit with your heels about two feet from your butt, holding a dumbbell at your chest. Keeping your back straight, lean your entire torso back a few inches. You should feel your abs working to keep you upright.
Without rounding your spine, slowly rotate your ribcage to the left, then return to centre to twist to the right. This completes one rep. Fewer reps done in a slow controlled manner will make your abs burn.