How do hotel room key cards work?

I was told that my card would expire at normal checkout time and they had to recode it to allow me a late checkout. Since it is only magnetic info on the card that can't "expire", how does the lock know to keep me out if I don't recode the card? They didn't do anything to the lock, so what... show more I was told that my card would expire at normal checkout time and they had to recode it to allow me a late checkout. Since it is only magnetic info on the card that can't "expire", how does the lock know to keep me out if I don't recode the card? They didn't do anything to the lock, so what are they doing to the card to let it work for 1 more hour?
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