All volleyball hitting and passing skills require some rotation of the arms. The rotation takes place in the shoulder, elbow and wrist joints. For example, in an underhand pass or dig you maintain the neutral position of the arms. When reaching out to the side with both arms they rotate to maintain this position. When diving for a ball you must laterally rotate the arm to get the palm side of the hand underneath the ball or medially to get the backside of the hand under the ball. In spiking, you laterally rotate the arm in the shoulder in the preparatory movements and then medially rotate prior to the hit and in the follow through.
Thus there is a great deal of arm rotation which involves the rotator cuff and other muscles surrounding the shoulder joint. To strengthen these muscles to assist in executing the different shots with the arms in different positions, and to help prevent rotator cuff injuries do medial and lateral rotations with the strength bar.
Hold the arm out straight in front of you gripping the bar at the non-weighted end and with the shaft perpendicular. Keep the arm straight and then rotate by dropping the weighted end of the bar inward as far as possible and up and over to the outside as far as possible. Go through a full range of motion to fully strengthen the muscles involved.
For side hits extend a straight arm out to the side and then medially and laterally rotate the arm. Do both arms to the same and opposite sides to produce different strength gains. The more positions you create to mimic the positions that the arm is in during different hits the more effective will be the all-round strengthening of the shoulder and arm muscles involved.
Note that if you bend the elbow the action will no longer affect the shoulder joint; it will be supination and pronation of the forearms. But, by keeping the arms straight and going through a full range of motion, you also strengthen the muscles involved in supination and pronation. Thus it is imperative that you keep the arm straight, the same position with which you typically hit the ball.