Robert J. Sawyer, a Canadian science fiction writer who has won a Hugo and a Nebula award, said, “General principles should not be based on exceptional cases.”

That is true, but bridge thrives on exceptions. Over the last two days, we have looked at the general principle for a defender’s overruff: Do so with any singleton, doubleton king-low, doubleton queen-low, and with tripleton queen-low-low unless partner rates to be short in the suit. But do not overruff with an honor that will still score a trick later and a lower card that might have been promoted as a winner.

Does today’s deal fit the bill, or is it an exception? West is defending against four spades. He leads his singleton heart. East wins with his queen, dropping South’s jack, and returns the heart 10. After South ruffs with the spade queen, how should West defend?

In yesterday’s deal, West started with K-10-x of trumps, and by not overruffing, his 10 became a trick. In that deal, though, partner could not have an immediate entry. Here, East has carefully led the heart 10, a suitpreference signal for diamonds. West should overruff and shift to a diamond. East will take two tricks in that suit, then lead another heart to promote a second trump trick for West and result in down two.

Note that if West does not overruff at trick two, South will probably cash his spade ace, then turn to clubs to discard his diamonds and get home. But, yes, he might not.

The key defensive point, though, is to overruff when your side has winners to cash in another suit.