Figure[Black to move]

First examine what Black attacks and how his pieces bear on White’s king. Either way you are led to the same point: his queen and knight both attack the bishop on f2. The next point to see is that this almost creates a mating attack with QxB; all that prevents it is the protection furnished by White’s queen on e2. When an otherwise good threat is frustrated by an enemy piece, focus on that piece and on how you might exploit the enemy’s dependence on it. When the piece is a queen, one way to exploit the situation is by attacking it: the queen’s value is so great that a simple threat against it may be enough to take the game. How can Black attack the queen? With his bishop, via Ba6. In addition to attacking Black’s queen this move pins it, preventing it from moving to another square on the second rank from which it still could defend the mating square f2.

The pin ends the game by immobilizing the queen. If White plays QxB, Black mates with QxB; if White plays anything other than QxB, Black mates on f2 anyway a couple of moves later. If White moves his bishop from f2 to, say, d4, hoping to use it to defend f2, now Black moves his rook up the f-file with RxBf3+; White is forced to bring his bishop back to f2 to offer a futile interposition, and mate follows with Qxf2 a move later. It's a study in the pin, of course, as well as in the art of attacking the guard.