Can someone explain to be how you devalue a currency "overnight" if everyone else follows suit and you're operating floating exchange rates?

I can understand how differential inflation rates eventually cause currencies to devalue or revalue against each other and I can see how global inflation will lead to a devaluation against hard assets like gold and oil. I just don't see how in a fiat system with floating exchange rate you can devalue by government dictat.

My question is prompted by a question on the Q-line this week which I don't think Jim really answered and others have asked before on the show. Apologies for being off topic but better we talk about something than spend our time having a pop at each other. Life's too short...