Our focus has been on attacks against the enemy king and queen when they guard pieces or mating squares. Attacks against minor pieces can be effective, too, though they are more complicated because in these cases it is less costly for your opponent to leave the guard where it is, exposed to capture, while launching a threat of his own elsewhere. A threat against a minor piece thus works best when accompanied by a larger threat—especially check.
After the relatively demanding positions we have been considering, the one to the left should seem easy. Back to first principles: you want to be aware of every point in White’s camp that Black has under attack. Here Black’s queen attacks White’s knight on e2. So you look at how the knight is protected and see the rook behind it. You can’t capture the rook; can you drive it away? Rooks flee knights and bishops, and Black has a dark-squared bishop that he can play to f2. If White leaves the rook where it is, he loses the exchange; if he moves it, he loses the knight—for there is no place for the rook to go where it will be safe and still protect e2. The root of the problem for White is that his rook is cramped against the edge of the board. If White’s knight-and-rook pair were in the middle of the board, the rook could retreat and still provide defense.