Nancy, biting the shit out of that latte straw now, grits that she actually cannot do that. Say those words. And when he asks why -- her feelings, not her thoughts -- they come out like they're in crayon. "...I can't. I'm afraid." And he knows damn well why, but he wants her to say it. This part's for her, not him. This is like the bathtub. Candles everywhere. What's she afraid of? "That you'll leave me. Us."
Andy rejoices, because that means he's got all the power. "Not you," he clarifies, making her smile. Like she cares about power, like it functions independent of her. Like it's something you do. It's not about him making the choice, or taking the power: It's about seeing who he is and knowing that he won't make the choice. It's about knowing him so well that you never do anything to send him away, even talking about the rules of the game as you're playing the game, if necessary.
Because going meta is the best way to fuck with boys, because they think about what's in front of them and they think it's real. They privilege their experience over anything you could possibly say: "I am going to break your heart," you say, looking them right in the eye, and they're like, "Sure you are." Works every time. "I am never going to love you," you say, "The way that you want me to." And they nod. And you say, "I mean it. I am being perfectly honest with you." And they nod. There is no center to this Tootsie Roll Pop.
But it's not just about a boy and a girl, because it's Nancy: It's about doors. He's asking Nancy to voluntarily close a door, when she's never closed a door in her life. Nancy is defined by leaving doors open, because she always knows where the exits are. So when they've jumped enough levels to actually talk about what they're talking about, it still comes down to Andy asking her to do the one thing that she cannot ever, ever do. Willingly chuck a resource; give up some of that domain. That's how get caught, pinned down. Dead. And letting him think he has the "power," whatever the fuck that means, is charming and indulgent and also completely irrelevant to the question.