Thanks for the sensible replies.
I suspect mine is considered to be one of the less sensible replies. But I stand by it.
Like I said, I'm only a controller. But the question seems to me like a controller asking what triggers the Short-term Conflict Alert system on his/her radar or what makes an ILS switch to the STBY tx'er.
We get trained about how the equipment we use works before we get qualified to work without supervision. Well, that's my experience anyway.
I'll retire quietly from this one now.