It goes against your grain, but sometimes you have to lose a trick intentionally. This often occurs when you are playing a No Trump contract and you have a long suit in Dummy with the only entries being in the long suit. Look at the following hand where you are South playing in 3 No Trump:
Opening lead: 5D
Count your winners. You have six, the AK of Hearts, the AK of Diamonds, the Ace of Clubs, and the Ace of Spades. But that leaves you three tricks short of your contract. How are you going to make this?
Your only chance is to set up your Heart suit in Dummy. But you've got a serious entry problem, since you only have two Hearts in your hand and no other entry on the Board. How to play it?
This is where you duck to set up the Heart suit. You must lose the first Heart trick. So the play goes that you take the opening lead in your hand with the King of Diamonds. Now you lead a low Heart to the Board. West plays the deuce. You play the six allowing East to take the trick! You are hoping for a 3-2 split in Hearts. East returns a Diamond which you take with your Ace. You then lead your last Heart to the AK on the Board. Hearts split 3-2 as you had hoped and you can run your five Heart tricks.
The only way you could make this hand was to lose the first Heart trick by ducking. That took out two of opponents' Hearts and allowed you to pull the remaining Hearts with your Ace and King when you got back in the lead. If you don't duck, you have no way to get back to Dummy without losing the lead when you try to take out the last Heart. If you lead to the Ace on your first lead of Hearts and then play the King, you are out of Hearts and North still has the nine of Hearts which will take the next Heart lead. Then there's no way for you to get back to Dummy to cash your remaining good Hearts.
Further, if you lead to the Ace and then lead a low Heart, you still can't get back because you only have two Hearts in your hand and the second Heart will go on your second lead. So you must lose the first Heart trick.
This is a fairly common situation and it's one where a lot of players misplay.
