It works the shoulders and core. Sit on the ball with your torso upright, draw in the abdominal muscles, chest should be lifted, shoulder blades pulled down, and feet flat on the floor. The bar should be held with an overhand grip just beyond shoulder width apart and held at clavicle level. Press the bar straight overhead keeping the biceps in-line with the ears and the barb directly over the shoulders. Don't allow the torso and ball to sway by keeping the abs braced. Keep the lower back arched naturally. Then return to original position.