The suits have a rank. Clubs is the lowest ranked of the suits and Spades the highest. We can use this fact to our advantage when we want to give partner information about which suit to switch to (or to lead). This the dummy:
The contract is 4S and partner leads the A of H's. This wins the trick and now partner has a dilemma: what to do next? If during the auction you had bid a suit (C's or D's) partner would presumably switch now to that suit. But there are many auctions where you don't get a chance to tell partner useful information about your hand. So let's assume partner has no idea what to switch to and so has to guess which minor suit to lead next. Ah yes. But what is it I keep saying? It's a partnership game!! Wake up! Why leave partner to guess when you - presumably! - know the answer. If you have the K of D's you want that suit led. If you have the AQ of C's you want that suit led. Right so how do we do this? Well, we use what is called a "Suit Preference Signal". There is no earthly point in partner continuing with the H suit so now the H card you follow with to the A will tell partner which suit to switch to. If you want a D, then discard a HIGH Heart - asking for the highest ranking of the other two suits. Alternatively if you wand a Club led then follow with a LOW heart , asking for the lowest of the other two suits (Obviously S's & H's are discounted) It is not always true that the card you follow with to the first trick is a SPS. It's just that in this case - nothing else matters. if you have the 269, the 2 asks for a Club and the 9 asks for a D. Of course you may be dealt the 234 of H's so how is partner to know what your signal means? I'm afraid they just have to work it out based on the discards from the dummy and declarer!
It follows therefore that when you are defending you must not blindly pull a card from your hand and place it on the table. What information are you giving partner? This communication is key in getting good scores. Do not give declarer tricks they do not deserve. Be frugal in your beneficence.