"Another heart calls...
Yeah I remember when we stole the night
We'd lie awake dreaming 'til the sun would wash the sky."

It's an incredible magic, unlike any of the sort he's handled before. It feels more solid, more real--because it's not just his, he realizes. Adding in the heart of another's magic, even if Rarity wasn't the active wielder, made it full of so much more life.

"Just as soon as I see you,
But didn't I, didn't I tell you"