An unknown person said, "Practical observation commonly consists of collecting a few facts and loading them with guesses."
At the bridge table, we sometimes have to guess what to do. More often, we can find the right play by correctly analyzing the evidence that is available. This is, of course, easier for the declarer, who can see his whole army, than for the defenders. But sometimes the defenders can benefit from having their hands hidden. If they cannot beat a contract by force, perhaps they can make declarer guess what to do. This deal is an example. How can the defenders give South a guess in three no-trump after West leads his fourth-highest spade?
The auction is as straightforward and practical as possible.
Declarer will play low from the board at trick one. Then East should see that if he wins with the ace, declarer is guaranteed at least one spade trick with dummy's queen, and perhaps two if he holds the king. So, East must put in the 10. Declarer wins with his king, crosses to the board with a diamond, and runs the club jack. West takes the trick with his king and continues with the spade jack.
We can see that South must play low from the dummy, blocking the spade suit and getting home with one spade, two hearts, three diamonds and three clubs. But suppose West started with A-J-9-5-3 of spades and East with 10-8-7? Then declarer's playing low from the board on the second round of spades would draw smiles only from the defenders as they take one club and four spades.
South has to guess.