Hearts trumps. It is known that B has king, queen, knave of trumps, and a losing spade or club—but uncertain which.
A leads the knave of diamonds. B trumps it.
Z should throw away a small trump, undertrumping B in order to keep two winning queens. If he discards a queen, he must do so at random, and perhaps throw away the suit of which B has the small one. By discarding his useless trump (which B would proceed to draw) he defers parting with either queen till after the next round, when the fall of the cards may assist him. B now leads a trump, and Y discards the losing club. B then leads another trump, and Z now knows that he ought to keep the spade. This case actually occurred in the presence of the writer, but Z, instead of undertrumping, discarded the wrong queen at random, and eventually lost the rubber in consequence.
For further illustrations of the Grand Coup, see Hands XXXVIII and XXXIX.
If the foregoing principles are reflectively perused, it will be seen that they mould the Theory of Whist into a harmonious whole. The Theory of Whist tells you how to play your own hand to the greatest advantage, how to assist your partner, and how to weaken and to obstruct your opponents; in short, it teaches how to take the best chance of making the greatest number of tricks. This knowledge constitutes a sound player. If to theoretical perfection you add the power of accurate observation, and of acute perception, together with a thorough comprehension of the whist capacities of partners and of opponents, you have all the elements necessary to form a Master of the Science.