Sometimes, a defender can paint a misleading picture for declarer, giving him the wrong impression of the hand by discarding – or not discarding – a suit.
I remember some years ago when Giorgio Duboin showed me a hand he played during an NABC that is a perfect example of such a situation.
You are West, defending against 6 Hearts from South, who is playing from the short side, after a 2NT opening.
You lead the Jack of spades. Let us look at the position from declarer's viewpoint.
Looks good, doesn't it? You will apparently lose only one heart (you can pick A10xx in any hand). So, the lead was the Jack of spades, and East followed with the 5. Declarer wins the spade lead in his hand and plays the King of hearts from his hand. You (West) have to make your first discard. The natural choice is a spade, but Duboin is at your side, and he whispers in your ear: pitching a spade is a big mistake.
Suppose you heed his advice and discard the Three of clubs. East ducks the heart trick (with A10xx). Declarer plays a club to dummy`s King, to play a heart from dummy. Partner ducks again, declarer wins the trick with the Nine, and it is your turn to make a new discard.
You still heed Duboin's advice and pitch a diamond. South plays the Jack of hearts, and still refuse to discard a spade, pitching a club. As expected, partner wins the Ace of hearts, and plays a diamond. Declarer wins the Ace in his hand and looks at this position:
Imagine you are the declarer. You saw West pitching a diamond and two clubs. Compare with your position in an alternate universe, where West pitched 3 spades, or 2 spades and a diamond. Would you change your line? How do you get to dummy to draw the last trump?
In actual play, Duboin defended as described, and was rewarded when declarer tried to get to dummy by playing a spade to the King. (He was protecting against 6-2 clubs, West having 6, while running the risk of 6-1 spades, West having six and having not pitched a single spade in the 3 previous trump tricks).