Source: ACBL 2007 Bulletins You can read Part I…Clicking Here
In this series, the importance of spot cards come home to roost. In bidding, you sometimes know that spot cards will be helpful. Here, however, is a case where spot cards are important in the play.
South plays in 4
after a simple value action, and West starts with the 
9. What are your thoughts?
It looks like you have two spade losers and you may have two diamond losers, too, if you’re not lucky. East did not bid, but it´s not impossible for him to have good spades and the 
A.
One line is to win the spade and draw trump. Give up a spade, hoping that they will divide 3- 3. If they do, you will have a good spade and can discard a diamond from dummy.
Can this line work?
Given that West led the 9, the odds are that East has the K Q J. He is likely to have four of them because a good West would not lead the 9 if he had the 9-4-2. If you’re not sure of this, take my word for it that leading the 9 or the 10 from three random cards is a serious losing habit. There are way too many holdings where leading an unsupported 9 or 10 will cost you a trick. Here is one example:
If West leads the 9, East plays the queen, losing to the ace. Now South can lead the 5 to the jack, which will set up South’s 8. It isn’t hard to show examples of hands where leading a 9 from this kind of holding is bad, and it is easier yet to show situations where leading the 10 (from 10-5–3, for example) is worse. Aside from the dangers of blowing a trick in the suit, you also run the risk of having your partner think you have a doubleton.
That is just one more part of the dark side of leading 9s and 10s with these holdings. Back to the play in 4 , you can
pretty much count on West having 0ne or two spades only.
It may be that you have to play a diamond at some time, hoping West has the ace, but not if you are watching the spot cards. You are missing the K Q J 9 4 2. If you cover the 9, East has to play the jack. After winning the ace, you draw trumps and continue spades. No matter what East does, he can only take two spades, which will set up a legitimate spade winner in your hand that you can use. The nice thing about this is that West cannot get in to lead a diamond.
On this hand, the key spot card was your 8. If you recognize that in time, you can guarantee a second spade trick by Covering the 9 with the 10. Some time ago, when I was writing an article on spot cards, I ran across this layout:
South is playing in a notrump contract and needs two tricks. Let’s say he leads the 6 from dummy and East play, the 2. South plays the 3 and West wins with the 10. When South regains the lead and plays the ace, the king drops, leaving South with the 9-8-7 and West the Q-J-5. South can continue hearts, setting up another trick by force.
Notice what happens if East covers the 6 with the king. South wins the ace, but this leaves West with the Q-J- 10-5 over South’s 9-8-7- 3. If West is patient, he will get four tricks.