At first, Fluttershy is startled. Why would Twilight be doing that? But she doesn't have long to wonder, because as soon as the necklace settles in place she has the strangest feeling. It's a bit like deja vu, and a bit like the spell Twilight used to bring her back when they first faced Discord, and a bit like waking up out of a very vivid dream, but not quite like any of those.

(It is, in fact, her true self being re-written over her changed destiny.)

It's over in a moment, and Fluttershy collapses to the floor. "Wha... what happened?"