This is aHand:

Bidding.

East-West reach yet another 3NT contract on this deal. West (declarer) has shown 12-14 points in a balanced hand containing five Hearts, while East has shown Diamonds.

Play.

Partner leads the ♣4. What prospects are there for the defense, after we play the ♣K and this holds the trick (indicating that declarer is unlikely to hold the ♣A)?

We decide to return partner’s suit. Yes, it might be right to switch to a Spade in case North holds ♠A x x or ♠K x x - but partner is unlikely to be impressed if he started with ♣A Q × × × or ♣A x x x x and an outside entry. 

Assuming that he has the ♣A, partner's Spades aren't likely to be all that strong, as there aren't enough points in the pack! There are several other possibilities (such as this deal) where a Spade switch would benefit declarer and not our side.

North takes the ♣A at trick two and clears the Clubs at trick three, declarer winning in hand with the ♣Q. The ♦10 runs to South, so we take the ♦K and lead..?

Well, it is just so natural to lead the ♠J that the card might jump out of our hand before the neurons in our brain had connected. What else could we lead? After all, West opened 1♥, dummy has the ♥K J and there are only two junior Spades on the table. Did someone not say: “Lead through strength and round to weakness�?

Well, maybe. However, consider the actions of our nearest and dearest (aka partner). He cleared the Clubs with the ♣2, the significance of which should not escape us. He clearly has a five-card suit and so, at trick three, has three equivalent Clubs. It is not necessary to go through the motions required to work out exactly what they are (actually the ♣9 7 2) - all we need to do is to appreciate that the ♣2 is North’s lowest Club. If partner had wanted a Spade return, he would have led a high Club – the fact that he has chosen the ♣2 means he wants us to lead the lower of the remaining suits.

Diamonds are out of the question, of course. We would no more lead one of those than stick our hand in a bonfire. The choice is Spade or Heart - and North has signaled for the latter. If we trust partner, we must grit our teeth at this point and lead a Heart (any Heart) and expect him to produce the Ace.

Glory be! Partner has what his signal says he has and two more Clubs complete West’s ruin. Two off. A Spade lead at trick five would have let 3NT make.