No, I am talking about the graph that displays the average contribution of the elbow to ball speed is 4%.
Right. And though it doesn't specifically say in the pitch speed section, if you go back to the injury section it is talking about elbow angle. I think (but cannot state to 100% certainty) that the 4% is referring to the angle of the elbow, mainly because that is the only measurable stat mentioned. The images on page 10 show the elbow angle circled, as well.