"Play the card you are known to hold" is a classic defensive maxim. Say, in a suit contract, declarer finesses dummy's Queen in a side-suit and cashes the Ace, discarding from hand. You, West, should play the King to keep the distribution hidden from declarer. Basic rule, but it has an extension less widely known: "... play the card you are about to be known to hold".

The setting is the semifinals of a knockout against a team better than yours. You are sitting East. Vulnerable opponents bid as shown below, partner leads the Eight of Hearts, bless her, and this is what you see ...

... after the opening lead ...

Declarer is studying dummy, so should you. By the time the 9 is called for, covered by your Queen and declarer's Ace, you should have a plan of defense. Do you ?... or are you ready to concede -1,430 / 13 IMP-s ? Can you see any way out ? Think !!!

Touch the Ace of Spades, when ready to proceed, declarer will lead that card next anyway...

... play the one you'll be known to hold, continued ...