This exercise works the obliques, the muscles on the sides of your abs.
Steps
Stand with your feet shoulder width apart, your knees slightly bent and your abs drawn in. Grasp a dumbbell in one hand and standing straight upright bend at the waist as far as possible to one side. Slowly return back up to the staring position. Switch the dumbbell to your other hand and repeat the exercise.
Tips
Be careful to bend only at your waist and not at the hips or knees.