Turns out it was a massage spell. Well that makes what Twilight did seem a little less disturbing. However, Twilight said she didn't want to lose her, which seems to suggest if Rarity wasn't put into a jar, her particles would be spread around and she would never return to her solid form, while still being conscious the entire time… Alright, back to potentially grimdark disturbing again.

Maybe it opens the other way?