In the Vest there are two body parts called the Left and Right Control Nodes. When you rotate one of David's Shoulders down, select the R or L control node and rotate it in the opposite direction. For example, after rotating David's arm down, look at the bend value on the dial to the right. Let's say for instance that it reads "-57.0". Select the control node and set it's bend value to "57.0". You can also do this with the "Front/Back" motion. Before you render, use the "Control Nodes Off" pose to make them invisible.