Source: www.suncoastbridge.com.au 
I’ve always said that you have arrived as a bridge player when you have learnt to routinely ‘count the hands’. This phrase means that a routine part of your play is to picture from the bidding and play the distribution and high card strength of the unseen hands. This isn’t easy, but it can be done, and becomes easier with practice. Of course your assessment may have to change as play progresses. But at trick 3, a good player will have a reasonable assessment of the hands.
#1 Take a simple example of counting. As West, you are playing 7NT on the J lead.
Success depends on a two way club finesse, which seems to be sheer guesswork. However, if you play out your other suits, you may be able to make a better informed guess. So you play the two major suits. North follows, but South discards a club on the third round of each major. Now you play on diamonds. On the third and fourth rounds, North discards hearts. Now you have an exact count of defenders’ hands: North is 5/5/2/1 with a singleton club and South is 2/2/3/6. Counting the hands has turned a guess into a certainty. Success is now assured. Play the K first (just in case North’s singleton is the Queen and then finesse the J which you know will win.)
Of course the cards aren’t always so obligingly distributed. Both pairs may follow to three rounds in the majors, but the cards may give you an idea of who has four cards in the suit. And someone will fail to follow the diamonds. So you will get an idea of the hand likely to have more clubs, and you will play for it to hold the Q.
#2 Another example. West is the dealer. He opens 1, passes round to you, and you find yourself playing 4 with West leading the A, then the K. Plan your play. You can see 5 spade tricks, and 4 tricks from clubs. If the clubs break, you get to discard a diamond from dummy, and ultimately a diamond ruff in dummy for 10 tricks. But if clubs don’t break, you can ruff the fourth club in dummy but are left with three diamonds in each hand. Where is your tenth trick to come from? The A may be on your right, but this seems unlikely on the bidding. Let’s see the distribution. You ruff the K and draw trumps, with both E and W following to two rounds. You then play on clubs, with West showing out on the third club and you ruff the fourth club. Now you know that West has only red cards left. Now the key play: play the J to West’s Q, discarding a small diamond. Now West is on lead, and must play either a diamond (giving you your diamond trick), or he must lead another heart, allowing you to ruff in dummy and discard a losing diamond from your hand. This is your tenth trick. This is the full hand.
The technique is called ‘loser on loser’ – a play you will be able to make fairly often if you keep count.
#3 Counting as defender: I hold this hand as East: J10x KJ8 xx Axxxx, and hear 1NT (16-18) from LHO, 2NT from RHO, and 3NT from LHO. Partner leads the 9. What should be going through my mind to this stage? RHO, to invite has 7 to a poor 9 HCP.LHO to accept has 17-18 HCP. I have 9 HCP. Partner will probably hold at best 5 HCP. RHO has not used Stayman, so he probably (but not certainly) does not hold a 4 card major. Partner’s 9 tends to deny an honour in spades Dummy goes down and this is what I see:
I play the 10 and South wins with the A. I know that the A is a false card, intended to confuse. West’s 9 suggests no higher honour. If the A were a true card, this would mean that West was leading the 9 from a holding such as KQ9x(x). He would lead the K from this holding. Besides, if the Ace were South’s only spade stopper, surely he would be ducking this trick? The K or Q would be a better false card from South. So now I know at trick 1 that South has the AKQ – 9 HCP, almost half of his total.
South now plays the A, then a diamond to the king, both of us following. Now he leads a low club from dummy. What do I know now? I’ve seen AKQ, and A, 13 HCP. The lead of a club is meaningless unless he holds the Q. This totals 15 HCP. He cannot hold the A, otherwise he would hold 19 HCP. Further he has already shown 8 top tricks, and this could be his ninth. So I know that partner holds the A. I rise with the A, play the K (South could hold the Q), then the J, and we take four tricks in hearts, and South goes one down.
Note that, if declarer had run a third round of diamonds, West may have been tempted to discard the 10 as a signal. He has now sacrificed the side’s fourth heart trick, and presents South with his contract. This brings up a useful tip: in defending NT especially, don’t waste a potential trick by trying to make a positive discard. Better just to discard what you don’t want and let partner work out where your strength lies. Here is the full hand: 
South was careless. He knows that he has to sneak his ninth trick before letting your side in. He should have kept his A hidden by playing a small diamond to the king, and then a small club from the table. As he played, he has revealed where his strength is. If you have to sneak a trick, do it early.
#4 Discovery play This is a hand that requires just that bit of extra imagination. West opens 1, North doubles and you play in 4, with East silent throughout. West leads the A, then K, then Q which you ruff, East following. You must lose the A, which, for his opening bid, you expect West to hold. So it seems that East must hold the K for a successful finesse and to make game. But here is where the expert will prove himself.
It can’t cost to find a bit more info before making your decision. So lead a club to the KQ. Somewhat to your surprise, East wins the ace! Now you can be sure that West for his opening bid must hold the K, so to make your contract it must be singleton. So you play to the A, dropping the king, with congratulations all round.
Here is the full hand:
This is called a ‘discovery play’ and was a clever play by South. But my admiration would go to the East player, who, suspecting South’s play, ducks the club trick. Now South will surely try the losing spade finesse. But what if South suspects East’s duck, and now plays the other club honour! A masterly cat and mouse game which I doubt anyone would find. When you become experienced in counting the hands, you will open up a new world of endplays.
Footnote: We all know the players who habitually hesitate when you are trying a finesse and don’t hold the finessible card. For example, you lead the J up to the AQ102 in dummy. And LHO hesitates suggesting he holds the K when he doesn’t. Or you lead the 2 up to KJ54 on the table and LHO with no honour hesitates. Not a blatant hesitation of course, but just enough to be apparent, but designed to mislead, and is actually cheating under the Laws, but hard to prove. Terence Reese in one of his books tells a story we could emulate in #1. If you suspect RHO, at trick 1 win the K on the table and lead the Q, suggesting that you may be finessing with a holding such as AJ109. If RHO hesitates when you know that he can’t have any reason, then mark him down as a suspect. When it’s time to play clubs, lead the 10 from the table. If RHO hesitates (meaning: he doesn’t hold the Q), win with the Ace and finesse the 9. If RHO plays smoothly (means he does hold the Q), then run the 10.
to be continued….