I point out how different the torso is positioned when they're at bat and how much power they get from their trunk by 'stepping in' to the pitch. It surprises them when I say they should have a similar position for pitching. Bend at the waist, and direct the front shoulder and hip towards the target. When the front foot lands, get the belly button over the front knee as quickly as possible. Then the trunk supplies the power, not the arm.
Shoulders are also in danger during a pitch.
Because the shoulder is the most mobile joint in the body, it is also the most unstable. The four muscles surrounding the shoulder, called the rotator cuff, are stretched during the acceleration and deceleration. Bones can shift, leading to damaged ligaments or pinched structures within the shoulder joint. Torn or strained rotator cuff muscles can result in permanent damage. To reduce the risk of injury, it is important that the muscles of the trunk and shoulder be strong, stable, and flexible.