Pain can be apowerful motivator but it is unreliable. If it is not reliable in the dojo, it will be much, much less reliable outside the dojo. In the dojo, you are dealing with people who are familiar with the specific pain being applied, and which direction to go in order to relive that pain. This is knowlege that we don't even realize we have and so assume that everyone will understand it automaticaly. They won't.
When teaching arrest and control (something I no longer do) we found that verbal instructions to the arrestee were necessary while applying any pain-conmpliance technique. Otherwise, unless they had experience in reciving that particular technique before, they tended to struggle even harder against the pain in an effort to make it stop. Furthermore, if, once cooperation had been established, the arresting officer did not reduce the pain level, the arrestee would renew his struggles since cooperation did not provide the immediate result desired from his point of view.