Source: Deceptive Card Play by David Bird & Marc Smith
Persuading a defender that he can defeat the contract by grabbing a winner can produce bushels of points. Suppose the defenders lead a club against a suit contract and this is your holding:
You would play the queen from dummy, hoping that West had led from the king.
Suppose instead that you hold a singleton ace in the South hand. Can you see any advantage in playing dummy’s queen now? You may convince the defenders that you have a loser in the suit. This can gain in more than one way.
Perhaps this is the full hand: 
You play in 6
and West leads the 
10. There are some legitimate chances. You could try to squeeze a defender who holds five diamonds along with either the K or four hearts. You can play for East to hold the 
A not more than twice guarded.
As you see, none of these chances materializes.
However, you can keep all but one of those options alive while greatly increasing the chance of a defender erring. Try playing the queen of clubs from dummy at Trick 1!
East covers with the king and you take the ace. What will happen, do you think, when you play a trump to the queen and lead a low heart?
East will surely jump in with the ace, thinking that he can cash the J. The Q will then provide a parking place for your diamond loser.
The same deceptive play may gain in this situation too:
Once more you are in 6 and West leads the 10 covered by the queen, king and ace. At Trick 2, you run the Q to East’s king. The chances are that he will try to cash a club trick rather than find the killing diamond switch. Note that it would not he a good idea to draw trumps before playing on hearts. West might then have a chance to signal for a diamond switch.