Twilight never meant to mesmerise Applejack, never intended her to be bound to her voice, forever vulnerable to a single soft word. Twilight would never take advantage of it. Except, she does need to understand the limits of the spell. And somehow, the limits of what Twilight will do keep shifting.
At least she'd never cast it on any of her other friends. That would be terrible. Wrong, to take more and more of her friends under her control. Definitely wrong.
Cover art cropped from 'Unresolved Issues...' by ScorpDK