Source: Jim Diebel for 
Let’s look at leads and the important role they play when defending against both no trump and suit contracts. Consider if you were the declarer in 3NT on the two hands below.
On Hand A, you receive the lead of the 
2 to the 10 and P J, and on Hand B you get the 
3 to the Q and A. How do you continue in each?
On Hand A, you have seven top tricks, you can easily establish two tricks in spades. With double stops in all other suits, you force out the A and K and chalk up nine tricks.
On Hand B, you were lucky to avoid a heart lead, so although you could force out the Q to attempt to get 10 tricks, you quickly cash out your nine winners. These are relatively easy plays to see as declarer, where all your assets are known, but on defense, it is much harder.
You have to assess whether to attack and attempt to develop extra tricks, or to cash out before declarer can take the tricks he needs.
You lead the 
9 against 4. East wins the Q, K, and you ruff the A with the 10 and declarer follows with the J. What now? It might seem obvious to try to cash the A, but upon reflection, this is unnecessary. Declarer has at least six spades and has shown up with three hearts. If he has a club, it can’t go anywhere. Exit with a trump and wait for the setting trick. Note that any other return will allow declarer to make his contract. If you has has hastly try to cash the A, declarer ruffs and his diamond loser vanishes. If you try to get out with a diamond, declarer can duck in dummy and eventually come to four diamond tricks.
[box type=”alert”]When you are defending a suit contract, do not attempt to cash tricks in suits that cannot disappear![/box]
This is often true in no trump contracts as well, but it is especially important against suits. Of all of the easily correctable errors I see, this is one of those things that lesser experienced players seem to get wrong over and over again. Defending accurately is difficult. To be a good defender, you have to know what declarer’s plan is going to be before he even starts it. On top of that, you need to recognize whether or not a particular hand needs to be defended aggressively or passively. How can you know? Most of the time, the answer will be staring you in the face. Look at the dummy. It usually will provide the necessary clues. Let’s look at a couple of simple examples. Suppose the auction goes 2 – P – 3 –the end
You win the 
A and now have to decide how to continue. With Hand A, there is no rush to do anything. You have the club suit under control, and you shouldn’t take a chance of developing anything for declarer. Return a trump. Declarer will ruff a diamond if he can, but you shouldn’t break the heart suit unless you are forced to do so. In other words, nothing is going away, so defend passively.
With Hand B, however, you don’t know who has the K, and even if partner has it, it can be finessed. An alarm should be ringing in your head that tricks are going to vanish if you don’t take them. It’s me to defend aggressively. Switch to the 6. Partner rates to hold the Ace, and if you’re lucky, he will return the Jack. If he returns a different heart, take your King, and try to cash one more diamond before all defensive tricks disappear on the club suit. When the dummy (or the auc on) suggests that there is a secondary source of tricks for declarer, the defense must aGempt to cash or develop their side suit winners as quickly as possible.
This principle arises oLen under many disguises. Suppose you are defending a heart contract, declarer is known to be out of diamonds, and dummy has been exhausted of trumps. You have the choice of leading the Ace of Clubs, or a diamond. Following our guidelines, it‘s clear to con nue leading diamonds. If declarer has any clubs, they’re not going anywhere. You can win it later. Furthermore, you don’t risk establishing a possible King of Clubs in declarer’s hand.
When defending, always stop to ask yourself before you lead a card: “Do I need to cash this card now, or can it wait? If declarer has any of these, can he pitch it somewhere?
Is there a danger of something going away?” If the answer to all of these questions is “no”, play safely and passively.