Figure 5.3.11.4[Black to move]

What does Black attack? His rook attacks White’s bishop, which is protected by two knights. His bishop on g7 attacks a knight. Put these together and you almost have something: Black could exchange away one of White’s knights with 1. …BxN; 2. b2xB; and then the bishop on e2 would be attacked once and protected once. That doesn’t achieve anything in itself, but it should cause you to turn your attention to the bishop’s other guard—the knight on f4—to see if it might likewise be vulnerable. Black has no way to capture it, but pieces at the outer edges of the center often can be chased with pawns. Black plays 2. …g6-g5, and the knight must flee; and there is no way for a knight to move and still protect the square it formerly did. Now Black has 3. …RxB and wins a piece.