Source: Master Play in Bridge by Terence Reese
A sure indication a declarer can have that he is up against top-class opponents is that his attempts to force out their high cards early will meet with failure. Most defenders recognise the advantage of holding up an ace when they sit over dummy’s K-Q, or of holding up a king when they sit over dummy’s A-Q-J. Most good players will go further, holding up the ace in the West position when declarer leads from x-x-x in dummy and puts on the king; if this play is made early in a notrump contract it is, of course, most unlikely that the king is unsupported. The object of this section is to persuade players that it pays to be still more daring in similar situations. Beginners are taught that high cards should be used to kill high cards, but there are many exceptions to that principle. Take this simple and common situation:
South leads the 2 and puts up the king from dummy. It is not necessary, in practical play, for East to put on the ace. If East plays low, South will duck on the way back and the defenders will still make two tricks. Of course, there may be tactical reasons why East should win the first lead; in the absence of such reasons, it will be better play for the defence to duck and mislead declarer regarding the position of the ace. Oddly enough, this type of play is equally effective when the defender has A-J-x.
After 1
-1
South bids 2 NT and North 3 NT. West makes the lead of 
J. South passes to the first trick, wins the second, and leads 3 to dummy’s Q. It is clear that if East wins with the ace, that will be the end of the defence, for after West has shown out on the third round of clubs South will set up the spades and diamonds without letting East on lead.
East can actually hold up his ace on the first round of diamonds with almost no fear of losing a trick. If East plays the 2 and follows with the jack South will play low from hand, playing West for A-x and East for J-10-x. If South’s diamonds are headed by the X-10, the defenders are not going to make more than one trick in any event. As the cards lie, then, East will hold the second trick and clear the clubs; South’s only chance of game will be to play another round of diamonds and he will finish two down.
There is one combination in which the hold up of the ace represents the only chance for two tricks.
If South’s lead of the 2 is covered by the 10, queen and ace, South’s only chance for four tricks will be to play for the drop on the next round; but if East holds up his ace, South will probably duck and lose to jack on the way back. To hold up a king in the blind position—that is, when sitting over the declarer—is more dangerous than holding up an ace when declarer has played the king. Nevertheless, when at notrump declarer finesses the queen of a suit in which he cannot have great length, it is generally safe to assume that he has A-Q-J.
When declarer leads low from table and puts on the queen, the expert game is to hold off. If South has A-Q-J-x, particularly, the hold up will spoil his timing and use of entries. Holding up a queen is still more hazardous, though this too can be most effective in match-point play.
When South, playing notrump, leads low and finesses the 10, it must, in the general way, be good play for East to duck. South will then assume that he has four top tricks in the suit and he may take a second finesse when dummy has no entry or when it is dangerous to allow East into the lead. Another nother time for courage is when a doubleton queen is one of the controlling cards in the enemy’s long suit.
South was in 3NT and West led 3. South won the first trick with A and led a club to dummy’s jack. Had dummy’s clubs been headed by the queen and East held the ace or king, the hold up would have been almost automatic. The queen in this position is the same, in effect, as the ace or king. It must be held up so that declarer can be exhausted of entries to dummy. South will surely finesse again and will be unable to get the clubs going.
Preserving a, Vital Entry
A defender who has a long suit and few entry cards must steel himself from the beginning not to let go his entries. On the next deal South opened 1NT and was raised to 3NT. West led 
3 and dummy played low. This was not the moment to finesse against the table, so East went in with the king and returned the 9, and South overtook the queen to win in dummy.
The natural play was to attack the entries of the danger hand, so declarer’s first move was to finesse Q. West had been expecting this and he played low. South now switched to clubs; East won with the ace and cleared the spades. South could have made nine tricks now by playing three rounds of hearts, but he was not to know who would win the third round and a repetition of the diamond finesse seemed the best chance. When West won with K, he was able to cash two spade winners.
Refusing to Win in a Critical Suit
It is generally good play, in defence, not to win a finesse on the first round when there is reason to suppose that declarer will repeat the finesse. A simple example occurs when declarer is induced to repeat an unprofitable finesse in preference to one that would have succeeded.
Playing in 4, South wins the club lead on table and finesses Q. If West wins, South will subsequently take a heart finesse and make his contract; but if West plays low, South will probably use his entry to dummy to repeat the spade finesse and will never have a chance to pick up Q.
The next deal is similar in general outlines, but it shows a different way in which the hold up may gain.
Once again South is in 4 and West leads J. South wins on the table and finesses Q, which is allowed to hold. Since East may have K 10 9, South cannot safely lay down A and his natural play is to lead a diamond to dummy’s queen. East takes with ace and plays a second club. South wins and crosses to Q for a second spade finesse. Now West takes K, puts his partner in with A, and ruffs a club return. A simple hold up in a side suit on the next hand made a difference of three tricks.
South dealt at game to North-South.
West made a good start by leading Q rather than 5. The old-fashioned lead is much more intelligent when declarer is likely to be short of the suit led. South ruffed the second club, forced out A, and had to ruff again when East led a third club. Having ruffed twice, South was down to the same number of trumps as West. After drawing these, he crossed to dummy with a spade and finessed Q. When West played low, South crossed to A and finessed again in diamonds; now West won with the king and put his partner in with J to make the rest of the tricks. It is clear that if West had won the first round of diamonds, that would have been his last trick, as he would have had no way of reaching his partner’s hand. South could have made the game, it is true, by continuing with 10 from hand, but that would have looked foolish if East had held the king. This was another hand on which a holdup caused declarer to lose control and to go down in a sensational way:
In a match in 1950 between a visiting American team and the Lyndhurst Club, South played in 4 doubled after East-West had bid 4.
West led A and South ruffed the second round. A finesse of Q may look like the most natural play, but since South would not be too well placed even if this came off (unless the diamonds were 2-2), South took a deep finesse of the 10. East could see that prospects would be poor for his side if he gave up diamond control, so he played the 3. Encouraged by this, South drew three rounds of trumps and finessed Q. Now the roof fell in: East won with the king and collected four tricks, to defeat the contract. If he had suspected the possibility of a holdup by East, South could have played more safely by taking the second diamond finesse after two rounds of trumps only. At worst, East would make a ruff that could be avoided; South would still make his doubled contract, losing one heart, one ruff, and one club. This is one more hand on which the deadly effect of holdup play is not obvious from a glance at the diagram:
West opened 10, and after taking one round of trumps, South crossed to K for a finesse of Q. Seeing a chance if South could be persuaded to finesse again, West held off. Ace and another spade would have won the contract now, but South, perceiving no danger, crossed to A and finessed again. West won and played a third club, on which East discarded his last spade. South was left with a spade loser and no way to dispose of it.
The Holdup with A-x-x of Trumps
A defender who holds A-x-x-x of the trump suit knows that often he must hold up his ace until he can exert pressure against the long trump hand. The advantage, on many occasions, of holding up with A-x-x is not so widely recognised. To begin with, fear that the trumps may be 4-2, when in fact they are 3-3, may cause declarer to make an unnecessary safety play. Below is a simple example in a part score contract:
West opened 1 and after two passes South protected with 1. North raised to 2 and all passed.
The defenders began with three top diamonds, and East discarded a club. West switched to J; South won and played two rounds of trumps, on which West refused to part with his ace. It was now slightly dangerous for South, playing in 2, to lead another round of trumps, for if West had A-x left, together with his two diamonds, he would be good for three more tricks. Thus the safe play for declarer is to abandon trumps and to push clubs, allowing East to make his low trump. In this example only an overtrick was at stake, but in pairs contests overtricks can be decisive.