Chuck folds the Prague situation into his speech -- she's turned her life around, and helped him do the same -- and creates the Eva Coupeau Foundation: Ivva will just be deciding how that $5 million gets spent, and that's the charity. But really -- and it's a bit of a gloss to get there admittedly -- but essentially he's derailing the narrative that Blair's working on -- pulling up the tracks before the train can get there -- by playing it Serena style: "Oh, you're afraid she's going to steal my money? How about if I just give it to her? Then you're a bad person and I'm not, and you're alone while we're twice as strong for this latest opportunity to show faith in each other, so thanks."
Anybody else, it would work. Marriages have been founded on lesser versions of this same idea. But you are dealing with Blair, specifically with her attempts to recreate her vivid imaginary life out here in the real world, and basically going right for her heart. Every spin she tries to put on it, he neutralizes, and not for romance or even secretly romantic reasons: So that he can say, and mean it, "I still know who you are down at the bottom, but you have no idea who I've risen to become." Which essentially makes her the scapegoat for his negative impulses, which is exactly 180 degrees from where they started. Not to mention the completely alien -- to Blair -- concept of defining yourself any other way than by the worst thought you've ever had.
I confess, I had no idea what Blair was going to pull and it was so awful that I really took my time putting it together, but this is where it starts: She gets the envelope of Chuck's effects back from Ivan, and then during Ivva's "speech," during which most people just looked awkwardly at the floor and nodded whenever it seemed likely, she's messing about with it in his suite. Afterwards, she asks to borrow Chuck for a sec and Eva awesomely goes, "Why not, Blair? What more can you possibly do to me?" Or rather, it would be an awesome line if you were saying it to anybody but Blair, but with Blair that's just a dare slapped across her face.
"What do you want now, to tell me Nate's running a secret drug ring?" All the improbabilities in that sentence, name them. "If he was, you'd probably make him the CEO of Bass Industries," B yells, and Chuck's like, "You're being gross, talk faster and get away from me." Blair says some pretty tacky things, dumb puns, just nasty shit, and he's counting down, and finally at the last second she blurts out that she's found proof Eva knew who Chuck Bass was long before he turned into Henry Prince, and thus has been playing him the whole time.