In the position on the left White has a possible capture in RxN. It's prevented by Black’s bishop on d6, which protects his knight. So that raises another question: does the bishop protect anything else you might therefore be able to take? Yes—the e5 pawn, which White can capture with his f3 knight. After he does, Black is stuck. He can forfeit the pawn straightaway or he can recapture with BxN—but this allows White to play RxN, still gaining a pawn. The Black bishop was overworked; it had too many defensive responsibilities. Whenever you see that an enemy piece guards more than one of its fellows, consider whether a blow like this might be possible.

It all looks simple enough, but complications lurk nearby. Suppose that after White’s Nxe5 Black moves his knight from b4 to c2: a zwischenzug (or “in between" move) that postpones the recapture and instead attacks White’s rook on e1. What would you do as White? It would be natural to move your rook, perhaps to c1 where it attacks the invading knight. Natural but wrong; for you must ask about Black’s next check and see that he would then have Nxe3+. Of course you just take his knight with your f2 pawn, but the priority of check has done its work: now Black takes your knight that still is sitting on e5—and there no longer is a Black knight on b4 for you to capture in reply.

What went wrong? When Black played Nc2, White should not have moved the rook from e1. Better just to leave it there and push forward again with the knight you used the first time—the one then on e4. With Nd7 White suddenly forks both of Black’s rooks, so now White can afford to let Black play NxRe1+. White replies with the recapture Rb1xN and then wins back his rook a move later at the other end of the board (Black only has time to save one of them). In the end White keeps the pawn he set out to capture.