Sometimes when the student finishes, they sometimes ask me about something other than the thing(s) I was focusing on. For example, maybe I'm really focused on their rhythm and they ask me about intonation.
For some reason, this makes me think of police speed traps. If I understand correctly, the radar gun can be in an "instant-on" mode where it sits there silently until some unlucky driver comes by going faster than some preset speed.
As I listen to a student play, I think I must have something analogous to that radar gun. Even if my attention is on phrasing or rhythm, a really out-of-tune note can come along and set off that radar gun in my mind.
That reminds me of a different tuner design that could be useful. The SmartMusic software has a feature in which the program listens to your note and plays it back ('in-tune' at least in the equal temperament world).
So, if we combine that SmartMusic feature with the notion of instant-on radar, we get a tuner that sits there silently as long as you are close enough to the desired pitch. If you stray beyond a certain amount (maybe 10 cents) the tuner starts playing the note.
I'd buy it.