You tell Mettaton that there’s a mirror behind him.

OH??? A MIRROR?? RIGHT, I HAVE TO LOOK PERFECT FOR OUR FINALE!

He turned around. You flipped the switch on his backside.

DID YOU. JUST FLIP. MY SWITCH? His screen starts glitching frantically, his hands pushing against the sides of his box-like form. A blinding flash occurs.

OH YES~

Ohhhh my. If you flipped my switch, that can only mean one thing. You're desperate for the premiere of my new body~