At twenty minutes Mione works it out, and she works it out because she has been watching Malfoid's face and not the bell.

MioneShe is deciding. That is the whole of it. She is standing on a rope deciding whether she wants to be in this house and the bell can hear her deciding.

CorvinaThen it will ring until dawn. I have watched it. A person can decide for eleven hours if nobody gives them a reason.

PorterThen give her a reason. Somebody get on that rope and give her one.