Yeah, the "Mini Lives" thread is definitely a better choice, but this is the Reader's Digest version:
It will be able to use a tuner from a regular Premiere, one day. But this won't be the case at launch. (needs software update at some point in the future.) It will be able to stream recordings from all Premieres on Day 1, but a 4-tuner box will still be mandatory for setup (initially, until the update is available).
Until said future update, "locked" tuner literally means dedicated for use by the Mini(s). A 4-tuner Tivo becomes a 3-tuner Tivo. It won't dynamically borrow and release until the future update. That's why a 4-tuner box is required initially... a 2-tuner box becoming a 1-tuner box would be a bad customer experience.
That being said, with regard to tuner activity, the Mini is expected to have some kind of standby state and/or be able to detect whether or not the TV is on from the video outputs.
Until the future update, tuner control is done from the host Tivo. You can dedicate 0 to 2 tuners to Minis. But this is moot in your case if you don't have a 4-tuner box.
The overall theme here for you here is waiting for the future update, and to check out the "Mini Lives" thread.