|Partner's Lead||Your holding|
No matter what card Declarer plays from Dummy, you MUST play the King. If you do not, then the suit is BLOCKED. Try playing out the hand at home. Deal out all the cards and see what happens when you play low verses unblocking the K♠. Here are the four hands.
Let's look at this from Declarer's point of view. How many winners does Declarer have?
♠ 1 winner
♥ 4 winners
♦ 3 winner
♣ 0 winners
Declarer has 8 winners. They need 9 tricks. The only way they can get another trick is by letting the Opponents in with the A♣. This will set up 3 more winners for Declarer. BUT this play also lets the Opponents in. When the Opponents get in they can take ALL their ♠ tricks.
Well they CAN if the suit doesn't get BLOCKED.
Scenario 1: Declarer plays low from Dummy.
If you also play low, Declarer will make their contract. What does Partner do? They play another ♠. Declarer plays low and you win the trick with your K♠. But YOU have no more ♠ left to return a ♠. Now it does not matter what you return. Declarer will win the trick and play a low ♣. Partner will win the A♣ but the ♠ suit is not established. Partner plays another ♠, Dummy wins with the A♠ and Declarer can take the rest of the tricks.
If, at trick 1, you play the K♠ this will win the trick. Now you need to RETURN your last ♠. You should now defeat the contract. If Partner is allowed to win the second trick, they will continue with a ♠. Now the ♠ suit is set up. Declarer can only take 8 tricks before they have to let Partner in with their A♣. Once in with the A♣, Partner can now cash all their winning ♠.
Scenario 2: Declarer plays the A♠ from Dummy.
If Declarer plays the A♠ from Dummy you MUST play your K♠ to unblock the suit. If you do this, what can Declarer do? They only have 8 tricks. They must let Partner in with their A♣ to get a ninth trick. But once they do this, Partner can play all their ♠ winners.
Let's say you play a low ♠ when Declarer plays the A♠. (That way you will win your K♠ eventually you tell yourself.)
If you play a low ♠ at trick one, watch what happens. Declarer plays a low ♣. Partner wins their A♣. This sets up all Declarer's ♣ winners. Partner continues on with a ♠. You win your K♠.
But you have NO MORE ♠ to play to Partner.
All Partners ♠ are good -- but you cannot get to Partner hand. You have to return another suit. No matter what suit you return, Declarer has the rest of the tricks.