White’s rook at f1 is unprotected and so is a target, but there is no way for Black to give check and attack it at the same time. Still, when you see any loose piece it pays to think hard about ways to attack it and create problems elsewhere with your queen. Here Black’s queen can attack the rook with Qc4, which also attacks the knight at h4. So examine the knight’s situation and notice that it is attacked once and defended once, and defended by a piece rather than a pawn—as good as loose, as we have seen elsewhere (you might just as well have spotted all this by regarding White's knight as a good target from the outset). Qc4 thus wins the knight, because after White moves the rook Black plays BxN and White dares not recapture. (If White replies to Qc4 with Qf2, protecting his rook, Black plays BxN just the same.)