In bridge, what are some exceptions to "second hand low?"

Question

Most beginners are taught to play "second hand low." That is, play a low card if one is led to them, to give the partner a chance to take the trick. This is particularly to avoid situations where second hand plays, say, a king, and it is taken by partner's singleton ace.

Like all rules, it has some exceptions. One is, play second hand high and grab the trick if you see a way to defeat the contract; e.g. running a suit in no trump. Another might be, if you let the opponents take a trick, it will be their game-winning trick (based on what you see on the board and infer from the bidding and play).

Are these valid exceptions to "second hand low?" What are other exceptions to the rule?"

Answer 1

The going up to cash your winners is a good reason.

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Another common reason to play second hand high is to deny an entry.

Here is an example.

South below is in 3NT, you are West and you lead a heart.

         AJT9x
         xxx
         xxx
         xx

Kxx              Qxx
QJTx             xx
xx               QJTx 
Jxxx             xxxx
         xx
         AKxx
         AKxx
         AKQ

During the bidding East/West know that South has no more than two spades (say the bidding goes 2C-2S-2NT-3NT starting with 2C by South, and East/West remaining silent).

South has 8 top tricks and needs one more.

South wins your heart Q with the K and leads a spade. At this point, you must play the K!

If you play low, declarer will play the 9. If East wins, declarer will make many overtricks (by repeating the diamond finesse). So East will have to duck this, but declarer will make his contract.

If you play the K, declarer can setup dummy's diamonds, but will not have an entry to cash them, and will only be able to make 8 tricks.

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Another reason to play second hand high is to prevent declarer from ducking a trick to a safe hand.

As an example:

         xxx
         xxx
         Kxx
         xxxx

AQxxx            Tx
Qx               JTx
xxx              QJTx 
Jxx              xxxx
         KJx
         AK9xx
         Axx
         AK

South is in 3NT and you are East.

Partner leads a low spade, and declarer wins your T with the J. (If he ducks this, he is down immediately).

Now declarer plays a diamond to dummy's K and plays a heart. At this point, you must play the heart J or T. If you play low, declarer will stick in the 9, losing the trick to the safe hand, which cannot continue spades without losing a trick and a tempo.

If declarer ducks your heart J, then you can push a spade through and beat the contract. So declarer will have to play the A or K, and partner will have to continue the good defence by dropping the heart Q! (else declarer can next play a low heart).

Since declarer has no more entries to the dummy, your Tx is safe from being finessed with the K9, and declarer will go down ultimately.

Answer 2

Another 'rule' new players are often given is to 'always cover an honor', i.e. if declarer is leading a queen towards dummy's ace, you are generally better off covering the queen with your king to prevent the finesse. After all, your partner may have the jack.

Similarly to the 'play low rule', there are also exceptions to this, e.g. when attacking entries as in Arybhara's answer.