Negative Hx

Hi, I'm new to game and just finished reading the rulebook pdf. I was wondering about this Hx. I understood that positive Hx means that the higher the score, better you know the person. But what really means that if you get negative Hx? Suddenly you know somebody less? What is the situation when that can happen? Like if I know somebody and she does something irrational and starts acting in a way that her actions are not predictable based on her earlier actions? What is Hx score when I meet somebody that I have never met before and I don't know at all, -3 or 0?

Hx is maybe the wonkiest part of the system.  There are many hacks for it. 

But it's useful to remember that in mechanical terms, what it really represents is your ability to help or hinder a person.  So one way of reading it would be: Knowing them well enough to predict the kinds of things you could say or do, that would either give them confidence or psyche them out. 

When you have negative Hx it might be like understanding them TOTALLY WRONG, like a Christian talking to a Satanist, or a Capitalist talking to a Communist...

But what really means that if you get negative Hx? Suddenly you know somebody less?

Exactly that. Or an easier way of thinking about it might be that suddenly you realize that you don't know them as well as you thought you did. Either they have changed, or you were mistaken, but in any case you have become more distant from them.