Both sides are mirrored. The magic has arbitrarily decided a front and a back. When Childermass flips the mirror over, he gets a lovely view of Strange's hand.

He can't help but perk up a little as Childermass mentions reversing the spell. That would make sense. There's already a framework, they'd have to do the complete opposite. Which granted, sounds like a pain, but it also sounds like something more doable than some other options.

"You'd have to teach me that one—I never learned anything like that." For obvious reasons. "But it does sound like a good idea."