If he followed along with the sheet music he would notice that she was coming up on a part where there were quite a few high notes - notes that most singers needed to use autotune to even hit without their voices crackling. Borderline opera notes but not quite there, but still pretty high. These notes she hit without hesitation at all, without voice cracking. And there was a flawless transition from those high notes, the climax of the song, back down into the lower notes once again.