Let's say you're West, defending against two spades.  You lead the king of hearts, partner signaling with the eight, continue with the ace and play a third heart, which South ruffs.

Declarer cashes the A-K of trump and plays four rounds of clubs, on which he discards two diamonds.  Whether or not you ruff the last club, South winds up making the contract with an overtrick, losing only a trump, two hearts and a diamond. 

Afterward, you realize that if you had defended differently, you could have done quite a bit better.  When partner played the eight of hearts at trick one - a signal which, in view of the bidding, obviously denoted possession of the queen - you should have led the three of hearts next rather than the ace.  

Had you done so, you would have been well-rewarded.  East would have won the queen and returned a diamond, allowing you to grab three diamong tricks and thereby defeat the contract.

The deal illustrates that a defender must always ready to adapt his plays to the situation at hand.  While it is true that a defender who leads the king from an A-K combination normally continues with the ace if he decides to play the suit again, this is by no means an invariable rule.

East's play of the eight in the present case, after having bid hearts, should be taken as a guarantee of the queen, and West should have no compunctions about underleading the ace at trick two in order to coax a diamond return from East at trick two.

Source: Steve Becker, Globe and Mail, March 4, 2025