There is more. Suppose your knight instead lands on f6, as pictured here. This is check, but not mate; Black can move his king to h8 or g7. But then when White moves his knight he will unmask a discovered check by his bishop. If you have read the section on discovered attacks you understand how potent a discovered check can be. In effect it gives the knight a free move because Black has to spend his time saving his king. Even in this skeletal position where few targets are visible, White thus would have Nd7+, attacking Black’s rook and then taking it, and winning the exchange, after Black moves his king out of check. So f6 is another powerful square for the knight, and it can get there from e4 or h5 or d7—three more squares on which we have added spots.

You can see where all this leads: if the enemy king is on g8 (its natural castling square) and your dark-squared bishop has a clear shot against h8, your knight can make terrible trouble by adding a check, and it can do this from almost any light-colored square on Black’s half of the board. The f5 square often is the knight’s most useful jumping-off point, but again your best actual options will depend on other features of the position such as how Black has defended those checking squares—h6, f6, and e7. The important thing to realize is how powerful a knight can become once you have control over the long dark diagonal. The beauty of the knight is that its power doesn't depend on whether any of the files leading to the king are open; thus the ideas described in these last three positions may be just as effective if Black has a pawn cluttering the g-file (which would be enough to frustrate a rook).