Source: THE SUN Seattle Unit 446 Newsletter February / March 2015
You are playing with a new partner in an IMP match. LHO opens 1NT, showing 11-14. RHO raises to 2N (invitational), and LHO passes, ending the auction. Partner leads the 
K and you see:
Assume partner has KQx. Declarer plays the 4 from dummy. Which club do you play? There is no clearly correct answer to this question without firm partnership agreements, which you don’t have in this situation. What you want is for partner to switch to a spade, either right now or after cashing the Q; you can then take four spades and four clubs for down three (+150).
However, from partner’s perspective, continuing clubs could be the correct defense (you could have five clubs and a side ace, while declarer may have eight tricks in the other suits if partner makes the wrong switch). If partner plays three rounds of clubs, you will be forced to cash the 
A to hold declarer to his contract! (Declarer must have the 
A and 
AK, the only missing high cards.)
If you play the 9 or 7, will partner read this as suit preference for spades, or encouraging clubs? If you play the 2, will partner read this as discouraging, or as a count card showing an odd number of clubs (perhaps five)? If partner reads your first card as discouraging but cashes the Q anyway (since he knows you have the A when the K wins the first trick), will he interpret your second card as suit preference? But what if you started with Axx and were limited in your choice of cards?
Experienced partnerships may have carding agreements that would define the answers to these questions. With an unfamiliar partner, or in a less experienced partnership, partner will be basically faced with a guess and may go wrong. I recommend that you eliminate partner’s dilemma! How?
Overtake his lead of the K with your A and return the 9! This sets up dummy’s J and partner, knowing that no more club tricks are available, will be forced to shift. Not only that, but the card you return at trick two (the highest outstanding club), when you almost certainly had a choice of spot cards for this play, shows suit preference, requesting a spade shift! Looking at that dummy, nothing else makes any sense anyway.
This defense guarantees two club tricks and four spade tricks for +50. True, you have cost your side 100 points if partner would have guessed right (for +150) without this play. But if he guesses wrong, the cost to your side is 170 points, the difference between -120 and +50 (although how much this costs in IMPs depends on the result at the other table). Personally, I think the odds that partner (even if an expert) will switch unless forced to do so are less than even money, in which case you are taking much the worst of it in any scenario. But then, you know your partner better than I do!