Billy is my baby!

In both cases, Max and Heather put their hands up close to Billy's face, and Billy either 1) catches Max's movement in his periphery vision, or 2) catches Heather's movement head on, but while in a dissociative state.

The way Billy catches these movements in his periphery or in a dissociative state is a sign that Billy's brain has been re-wired because of abuse. We all have an exogenous orienting response, which is an involuntary shift of attention to a sudden, unexpected action (movement, sound, flash of light, etc.). It's a survival based response meant to help us react to potential threats. This response is heightened and overstimulated in abuse victims, keeping them in that constant state of hyper-vigilant survival where unexpected actions, including ones that are not intended to be harmful, are perceived as a threat. It is important to note that however harmless the intent was in both of these scenarios, Billy cannot distinguish gentle or helpful movements/touches from violent and abusive touches because he has never known touch to be gentle. He has no basis by which he can determine touches to be violent or gentle. Hence his highly reactive state.

The moment Billy catches the movement in his periphery, or even in a dissociative state, he enacts the exact same response: he abruptly catches Max and Heather's wrist and scoops their hands forward in front of his face so that he can have a clear line of sight of their hand. In the scene with Max, this movement is nearly simultaneous to his redirected line of sight on Max's hand first before he looks over at her. (He looks to her hand first because the hand is the perceived threat.) With Heather, he doesn't have to redirect his line of sight because the movement is happening right in front of him. Still, Billy catches, scoops, and lifts Heather's hand up right in front of his eyes because even though the action is done right in front of his face, Billy's not fully there- he's dissociating.

The need for Billy to have a clear line of sight of their hands (and to have that clear line of sight FAST) is yet another trauma response. It's Billy trying to see what their hands are doing. He has to see the thing that his mind and body automatically perceive as a potential threat. If he can see it, then he can try to mitigate it. It's Billy trying to take control of a movement that he, in his mind and body, perceives as a potential act of violence. Again, remember, Billy sees movement near his face and instinctually reacts based on a lifetime of abuse where movement and touch around his face is violent.

There's also something else worthy to point out, and it's similarities and the subtle differences in the rest of Billy's body language with Max versus with Heather.

Similarities

Take a look at his facial expression when Max and Heather's hands get near his face. There's no surprise in Billy's eyes or on his face. There is resignation, and there is (more so with Heather) a numbness. Why? Because Billy is accustomed to people hitting him. Movements near his face are normal. Expected.

Differences

With Max, we can see that Billy's body is carrying tension, agitation, and anxiety. It's all pent up in his body because he just watched Max and Lucas get into an argument whereby Max was visibly upset and raised her voice against Lucas. He has just been witness to a verbal fight between Max and Lucas, one that Billy would immediately 1) catastrophize that Max is in trouble and 2) know that Max's actions and choices will wind up with Neil beating him. The tension in his body carries these fears and anxieties. He is in survival mode. And when he's in survival mode, it is hard to come down from it.

We don't see this in the gif above, but when Billy says, "Hey. This is serious shit, okay?" there's a tremor on Billy's face. We see Billy's body tremors and shakes in other points in the series, which you can read about in depth here. He displays body shakes/tremors because he can't put this fear and anxiety anywhere except within because he cannot safely emotionally regulate. He lives in an unsafe environment that prohibits him from being able to emotionally regulate or heal (and we know that Neil has repeatedly tried to beat Billy out of showing emotions like crying or fear). Billy keeps so much of this inside so that it builds and builds and builds, creating a tension and intensity in his body language until something triggers it. That's when we start seeing that fear and anxiety manifest through his body shaking.

With Heather, it's entirely different. Billy's body is completely numb and deadened. In this moment, it's about nobody else BUT Billy. There are no other outside stressors here as there are with Max and with what Billy fears awaits him with Neil. And it's important to understand that after years of abuse, sexualization, and SA paired with years of Billy trying to reach out only to realize that help never comes (read more here), that creates a line of thought within Billy that he's not really worthy or deserving of help. When it comes to the care Heather is trying to give to Billy (i.e. tend to his wounds), there is this implied sense that his body has simply given up (read more here).