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Sometimes declarer will lead a long suit from one hand, for a "ruffing finesse" in order to ruff with a void in the other. If the intervening opponent ruffs with say, the 9, the declarer might "overruff" to win the trick (and capture the opposing trump). If this opponent ruffs with the "master" trump, declarer might discard from a side suit instead.

If the declarer had ONLY trumps left, he would have to "discard" a trump (underruff) if the opponent ruffs with the master trump.

But there are times when the declare will "underruff" even when he can "overruff," or at least has a side suit card to discard. Why would that be? Would it be to "throw in" an opponent? Or might that side suit card have "strategic" value, such as for "transportation" to the other hand? How would that outweigh the value of the "wasted" trump?

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Is the question mainly about underruffing be declarer? or do you want to consider underruffing by defenders too? –  Aryabhata Apr 29 '12 at 18:20
    
@Aryabhata: Either. I'd like to hear your experienced opinion. –  Tom Au Apr 29 '12 at 22:00
    
I have updated the answer with a few more examples. –  Aryabhata Apr 30 '12 at 8:09
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2 Answers

up vote 10 down vote accepted

Here are some situations where underruffing might be required.

1) Execute a Throw In

Consider the following situation, where spades are trump and south is the declarer. The lead is with West and he leads a diamond.

    -
    xxx
    -
    -

-         KQT
-         -
AKQ       -
-         -
    AJ9
    -
    -
    -

South holding the AJ9 of trumps needs two of the last three tricks.

West is on lead and leads a diamond and east holding the KQT of trumps ruffs with the Q.

Now South must underruff with the 9, forcing East to lead from the KT into the AJ.

If South overruffs with the A, then South has to lead from the J9 into KT, losing two tricks.

2) To avoid a Squeeze

On defense sometimes the defender is forced to underruff (discard a trump) to avoid being squeezed. This might well apply to declarer (but is probably unlikely).

For an example for defense, consider the following example:

          Txx
          KJxx
          xx
          KJxx

x                  xxxx
T9x                Qxx
KQJTxx             xx
T9x                Qxxx

          AKQJx
          Axx
          Axx
          Ax

You are East. South is declarer in small slam in spades after West preempts in diamonds (and showing 0-7 points in your system, just for the sake of this problem).

West leads the DK which declarer ducks and west continues with the diamond J which declarer wins the A. At this point declarer has 11 tricks, and knows you hold both the HQ and CQ.

Now declarer plays AK of trump, and then ruffs a diamond in dummy with the T.

At this point, you must now underruff! If you throw a heart, declarer can draw trumps throwing 2 clubs and 1 heart from from dummy, play A, K of hearts and drop your HQ, setting up the HJ.

If you throw a club, declarer can play AK of clubs, and ruff a club, dropping your Q, with the HK as entry to the good CJ.

So you underruff with a trump. Now declarer can draw trumps, but since you discard after dummy, you can just follow his discards and avoid being squeezed.

3) Executing a Trump coup

Then there might be trump coup situations where declarer/defender is trying to shorten the trump holding (to avoid being on lead at the wrong time) and might choose to underruff.

For an example on defense:

           xxx
           Qxx 
           xxxx
           Kxx
Txxxx              -
xxx                Jxxx
Txx                QJx
xx                 QJxxxx

          AKQJ9
          AKx
          AKx
          Ax

You are west. South is declarer in a small slam in spades. You lead a heart which declarer wins in hand. He plays the trump ace, and gets the bad news.

It looks like declarer now has two unavoidable losers, a trump and a diamond, but declarer tries his best.

He plays one more round of trumps, cashes his hearts, and plays the A and K of clubs to reach this position.

           x
           - 
           xxxx
           x
Txx              
-                   Immaterial
Txx                
-                 

          QJ9
          -
          AKx
          -

Declare now plays a club and ruffs it with the Q.

At this point, you must underruff!

If you throw a diamond, declarer can play diamond A,K and low diamond. You are left with nothing but trumps and are forced to ruff that (basically ruffing your partners trick), and then lead from the Tx into declarers J9, allowing declarer to make the small slam!

But, if you underruff, now when declarer plays diamond A, K and low, your partner can win it, and then your T becomes the setting trick.

So underruffing is not unheard of, and is sometimes required. There are probably more situations than described above, but I believe they(including 2 and 3 above) tend to appear rarely.

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2  
Another case when declaring is when you need an entry from one hand to the other and you choose to underruff to retain that entry. –  Mark B Aug 8 '12 at 22:05
    
@MarkB: True! Reminds me of a hand played by Benito Garozzo. I might add that later... –  Aryabhata Aug 9 '12 at 3:49
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The first answer is good but I also want to point out a situation where it doesn't matter if you duck now or later:

  • your partner and one opponent are out of trump
  • the other opponent has 2 trump remaining that are in sequence or close to it, (i.e. 8-9)
  • your hand is right after the opponent with 2 trump and you have 1 trump above the sequence and several trumps below it

Under these conditions, you already know that your opponent will win one of his trumps and lose the other. So in some cases you may not be sacrificing anything by underruffing - just timing when you want the loss to occur.

With this timing option may want to lose earlier rather than later to set up running a long side suit without interference or some other sequence that requires you play a lot of cards in a row with appropriate transportation (not getting stuck in your hand).

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