Hx is more about the Players then their characters. Consider the actual mechanics that operate out of the stat. Hx represents how close one player is to another's and changes the dynamics of the scene based on who can best help and who can best get in the way. If you have you players whose characters have a falling out, they might try to distance themselves from each other. The -1 Hx option provides them a way to signal to the other player that there is growing tension between the characters.
Likewise, the ways in which to gain Hx help the player mechanics by virtue of rewarding certain behaviors. Taking damage increases your Hx, makes you better able to handle a hostile PC, thus taking damage isnt just a negative and can be seen as a "at least I got" something out of the exchange. Likewise the Hx reset provides ways to continue to facilitate character growth, healing a character in need... sharing something with them... rewarding them at the end of a session to show that you're character is getting closer to theirs, or at least is appearing to be more open.
The benefits of Hx is that it Complicates the interactions between characters. You could let another character sit at 3 hx for you, just so they can help you out when you need it. But maybe giving them the experience helps motivate them to continue to help you out. Its a trade, a tension, a signal, a complication. See AW has a tendancy not to focus only on a single player's ownership over their character, but as a collective ownership of the entire narrative by the entire group. Hx is a way to socialize the context of the character's relations for the players. If someone is sitting at 3 hx with you, and they piss you off to the point you want to pull out your gun and just shoot them in the head right then and there... it might be risky since they're clearly going to try to interfere with your success... which makes it much harder for you to shoot them in the head right then. Maybe you need to give them another experience first, get closer. Or take some experiences away, giving the other player some /time/ to digest and adapt to the narrative.
Of course just shooting them will also give them hx to lower their further interference, at least for the next roll... but they also get exp out of it. So there is complication. Its confusing, because its not entirely the literal HISTORY between characters, obviously that doesn't go away. But it can help reflect some of the more hard to detail and changing facets of the characters... Emotion (Sometimes things get under your skin, something your believe things as you want them rather then as they are), Perception (you like to think you know your friend, but that doesn't mean you always know where you friend is coming from) Attention (sometimes you've been focused on something else, like... maybe getting the thing you bought with your exp...)
It benefits characters to give each other Exp, and since there is a maximum of one per week... it means a player might have to make a hard choice (or pick a side) by giving hx out or taking it away. AW is about testing the character and player within the scarcity of things, even the scarcity of teamwork. Not saying it will always do these things, but it can be a very neat indicator on how players perceive their own characters (most important) interactions in a given session.
HX is Fickle.
If you really want an IC way of expressing the idea, try this one on for size:
No one is ever actually closer to someone else, its all a matter of degrees. You can push someone away just as fast as you can pull someone in. You can pull them in close to stab them in the heart, or to bear yours and weep on their shoulder. Hx doesnt represent KNOWLEDGE at all. It represents the characters fickle focus of attention.
Hx is the building of experiences. When they manifest as a moment of your affected history (hit 3), mark an experience circle. This resets the Hx "clock" it has no bearing on the closeness between you and they, but those experiences have culminated in something, and that something is a peak. Afterwards your attention moves on to other things, and it takes awhile to refocus back on that person.