In defense, every action taken – even if it is done for the right reasons – can provide important information to the declarant. Good defense requires you to hide the important stuff.
West led Q ♦. Declarer needed one of his two finesse to succeed; either a shovel or a stick. He dropped the lead, winning J ♦ continuation with dummy’s K ♦ and leading 4 ♠. When East played low, South Q ♠ inserted; West dropped 2 ♠. Publisher returned to dummy with A ♦ and repeated the digging finesse. When West won K hierdie this time, declarer frowned. Now he had no access to dummy to try out the club finesse. The hand ended in frustration, with just eight tricks. As the results came in, many other South players signed their 3NT contract. . .
The key to the hand was that West refused to tell Explainer to lose the graffiti. By diving the first shovel, declarer accepted that East K ♠ kept and used the last diamond entry to dummy to repeat what was actually the lost finesse. If West K ♠ had won in her first innings, Declar would return to dummy with his top diamond and now try the club finesse. Since it succeeds, so does the contract. The withholding of information was the making of the defense.
In simple terms, against a no-trump contract, if the declarant takes a finesse you expect to be repeated, not winning the first time. It keeps the declarant exactly where you want him: in the dark.