Johnklets:
The idea of spanking on the bottom, I believe, was based on the assumption that one can inflict pain without injury. I also believe that this was not done with exposing raw skin. I am aware of situations in which the intensity and amount of petch given was enough to cause serious pain of long duration. And I know of one instance that the rebbe who did that was terminated immediately and banned from chinuch for life. Yes, the powers that be kept track of his efforts to be rehired elsewhere and insured it would not happen.