The key protects you...

The key's power over things of fiction allows Robbie to perfectly resist the mental control powers of the lasso. But also slows down time to give him a chance to think about your next move. Robbie had to choose if he resists the enslavement and then try to get the Uberbitch back. Or pretend to fall under her spell until such time that he could make his own mind control move.

He has to choose quickly as he didn't know how soon this slowing effect can go on?