Figure[White to move]

This position only sort of fits in this section, but it's instructive nevertheless. On a8 Black has left a loose rook with an open diagonal running toward it. White has no good mating threats but his knight is in an intriguing position: it attacks e7, from which square it would check Black’s king. Be alert for forks anytime the knight is one move from attacking the king. Meanwhile White looks for ways to threaten the loose rook and sees that one of them is Qc6, which attacks both rook and queen and requires a reply: perhaps QxQ. Ask how the board would look after that small sequence, and notice that Black’s king and queen both would be on light squares—ready to be forked with NxB, which wins the bishop for White when the smoke clears.

Now don’t forget to ask whether Black’s queen, when attacked by White’s queen, could both move out of harm’s way and defend the other attacked piece. Suppose, for example, that instead of playing QxQ, Black plays Qb8. Now White’s queen can’t take Black’s queen or rook. But—aha!—the bishop at e7 would be left loose by the queen’s movement, so White could play NxB with impunity. If Black instead had played Qd8 so as to protect both bishop and rook, White again envisions NxB. Black could answer with QxN; but then the rook would be loose (Black can’t have it both ways), and QxR+ takes it.

So the result is that Qc6 is a working queen fork of Black's queen and rook—though this time without a mate threat in the background. Instead it works because Black's queen is vulnerable for a bunch of other reasons: it can move only at great cost, because it is doing other defensive work (guarding the bishop and rook) and because if it captures on c6 it lays itself open to a knight fork.