Basically the reader writes data back to the fob and expects to see it the next time it reads it. This results in either the original or the copy getting de-synced.

One interesting capability that this unlocks is that battery powered, offline readers (think apartment door that uses the same fob as the lobby) can write out things like battery state so that apartment maintenance knows when it's time to swap out batteries.

This is one in SF. I can clone the card, and it'll work, but then within a few days it won't work any longer.