If the flop comes 6♥ 7♥ 8♥, Carol has a lock on the high hand with her 10-high straight flush, but Alice and Bob are still competing for the low half of the pot.
On the other hand, on a flop of 8-7-6, a hand containing A-2-3-4 is considered counterfeit-proof, because it currently is the nut low (8-7-6-2-A), and even if an A or a 2 arrives on the turn, it will remain the nut low (7-6-3-2-A) and will remain protected from counterfeiting on the river.
Often Texas hold 'em counterfeiting is less likely to cause a player to lose an entire pot.
The A-K player's hand didn't get worse in rank, but it becomes less likely to win the entire pot.
This other player could win, for example, with A♠ J♦, since all that is needed is one card, the A♠, to complete the nut flush.
In common usage, the term counterfeiting is sometimes applied in a non-standard way to situations where hands do change value.