I think this is a great question and one that I would be very interested to hear a surgeon or other qualified clinician explain. In modern AN surgery virtually every procedure involves nerve monitoring, yet facial nerve damage still occurs. In my limited understanding of how the machines work, they are picking up a signal from the nerve but they are not that exact to insure no damage has occurred. As long as the nerve is intact it will transmit a signal to the monitor, however any nicks to the nerve can cause a temporary or permanent deficit. I would also suspect that any change to the monitor signal may be already too late as opposed to a early warning to "back off". I do not believe they provide an early warning that the surgeon is "close" , I think they react when the nerve has been impacted. All this is best guess on my part and I freely admit to being beyond my knowledge base in this area. Hopefully someone can get a more qualified answer.
Mark