This position is not an especially good fit for the current section because it does not involve a discovery in the usual sense, but it goes here as well as anyplace. White has to do something about Black’s threat of BxR. He looks at any captures he can make and considers their consequences—what lines they would open and so forth. There is just one: BxB, resulting in RxB. The interesting feature of this exchange is that it not only extinguishes the threat against White’s a1 rook but also leaves that rook and White’s king with nothing between them—and an enemy target on b2. White therefore can devastate Black by castling: 0-0-0. Suddenly Black’s king has been checked by White’s rook, while Black’s rook is about to be taken for free by White’s king. Castling into an attack is not something you have a chance to do very often, but when the opportunity arises it is memorable.