'me' or 'you' nevermind, the point is still. How to differentiate if she wants your attention vs seeing you as a potential branch she want to hang on? How does that process occur?

If someone here figured that their girl is giving attention to someone, they would break up with her, right, but, in what moment women break up with their boyfriend to jump on second prospect?

Is there any 'rules' or explanation to this? What's the point of doing it anyways?