Cards speak (poker)

Cards speak (for themselves)... At the showdown, players will lay down their cards, so revealing the contents of their hand. In this way, unlike declarations, a player who might not have seen a particular combination in their hand will have the benefit of further examination by others.

In this way, the cards are said to be speaking for themselves - it doesn't matter what a player declares, the final decision on the winner of the hand is made by inspection.