What captures are available to Black? BxN and QxQ. Both queens are protected, but that is only an invitation to probe deeper into the strength of the protection. White's queen is guarded by his knight, and the knight can be taken. Thus if Black plays BxN, White’s queen suddenly is bereft of protection. Now of course White would not be inclined to play the recapture e3xB, permitting his queen to be taken by Black on the next move. Instead he will send his exposed queen on a suicide run: QxQ. But then Black has BxQ and wins a piece with the sequence.

Notice the delicacy of it all. Here as in the last case, the sequence works only because the bishop that removes the guard continues to protect Black’s queen against QxQ, thus enabling Black to both make the recapture and evacuate the bishop from danger with the same move. The lesson repeats: when you are removing the enemy queen’s guard, you have to think carefully about the danger that he will reply QxQ; make sure either that he cannot do it or that if he does, you will have a recapture that does not leave one of your other pieces in the lurch.