I was commentating on BBO when this deal arose from the 2017 European Championships:
South opened 1NT (15-17). I see only 14 HCP, but as frequently happens, players upgrade for good 5-card suits. Perhaps the abundance of queens and jacks should have prevented such optimism.
Anyway, game would have been reached regardless.
After 1NT, North transferred to hearts and then offered a choice of games with 3NT. South chose 4 and West got off to a good lead, a low club (this pair leads low from xxx). When East doesn't double the 2 transfer, that is a slight nudge away from a diamond lead and makes a club a slightly better guess (leading the A or from it is never an option; a trump is possible).
Declarer played low from dummy and East won the K. Now what?
From our catbird seat, it is easy to see 3 more defensive top tricks. But how would East know to dangerously switch to spades? Declarer could have Axx or even AJx where this would be costly. Instead, East returned a normal 10 (maybe partner had Qxx). Declarer won, West playing the 6.
Declarer led the Q and West played the 9. What's with the unnecessary high club and heart spots?
Expert defenders use lots of subtle suit-preference signals. Since those club and heart spots make no sense for other uses (like count or attitude), they should send a message about the other suits (spades/diamonds). West's high cards in both cases, screamed spades. East got the message and (just in time) switched to spades (upon winning his A) for down 1. Had East stayed passive, declarer would have made an overtrick (throwing dummy's spades on his good diamonds).