By now you get the point: when you see a relative pin like White has here against the knight on f6, consider the consequences if the knight breaks out of the pin and starts an attack against the king at the other end of the board. Here Black has 1. …NxN, inviting White to play 2. BxQ. If he does, then next comes 2. …Bb4+. White has to interpose his queen, and Black has the familiar gain of a piece: 3. …BxQ+; 4. KxB, KxB. For some more exploration of this general motif, see the chapter on the relative pin (and the discussion of how to break out of one).

Finis.