White’s knight is on a dark square, along with Black’s knight and king. There is a potential fork at f5, but the g3 knight is pinned, f5 is protected by both queen and knight, and anyway the fork is of no use because it ends up trading a knight for a knight. So it’s only the barest glimmer of an idea, but when you ask whether you can capture the d4 knight with something else (and what happens then) you find 1. QxN+, QxQ; 2. Nf5+, a knight fork that wins a piece.

Regardless of whether the fork occurred to you at the start, consideration of QxN+ would have been mandatory because it was one of White’s three possible checks. We keep emphasizing these separate paths to the solution—i.e., showing how it might have been found by looking at checks and their consequences—because in many positions the idea isn't as clear at the outset as it perhaps has been in these early positions; and in that case starting with a look at your forcing moves is the best way to find tactical ideas. In this case, after imagining QxN+ and the recapture QxQ you reevaluate your knight's tactical prospects. You notice that your knight and Black’s queen and king all would be sitting on dark squares (and the knight would have become unpinned), calling for the fork Nf5+.