I'm not sure I'm explaing my question well so but if you take the pinball example as a point of reference, the flipper specifically.  There are 2 component parts, the flipper, the joint and the holder.  You configure physics objects on the flipper and holder and a joint on the joint.  I had thought I understood that the holder determined where the joint was but I find that to not be the case and in fact I find that I can place the holder anyplace on the canvas and it has no affect on the behavior.  I am not guessing that the holder is really a placeholder and is only needed to give an link between the joint and flipper.  Am I correct or is it more than that?