White’s most advanced knight (generally the one you want to examine first) is on a light square. Again you might just look for knight moves, or you might look for forking candidates by scanning for Black pieces also on light squares, and find many—both of his knights, one of his bishops, one of his rooks, and his king. Nd6+ forks Black’s king and b7 bishop; the bishop is unprotected—is “loose”—making it a good target. The next question is whether the square you need (d6) is protected. It is, by Black’s bishop at e7. But then consider how the board would look if the bishop moved to d6 to take the knight. See that Black’s queen would then be taken by White’s bishop at g5; in other words, Black’s bishop is pinned to his queen. Nd6+ thus wins Black’s b7 bishop without fanfare.
There is another point to consider here. You want to think not just about what your tactical moves will achieve in the way of material gains, but also about how the board will look after the sequence you want to play. This point applies to all tactical operations; we will encounter it constantly. The important point here involves the work that your e4 knight is doing before it is sent off to inflict a fork. It's guarding the bishop on g5. To be more precise, at the start of the pictured position the bishop is protected twice (by White’s two knights) and attacked twice (by Black’s bishop and the queen behind it). The bishop therefore was safe: if Black captured it, White would recapture; if Black captured again, White would recapture again. But when White sends his knight off from e4 to d6, the bishop loses one of its guards. While this doesn’t matter so long as White is keeping Black busy with checks, notice the hazard that arises once White plays NxB at the end of the sequence. His bishop back on g5 now is attacked twice and defended only once. Does he lose it? No—but only because once his knight ends up on b7, it attacks Black’s queen. Now if Black plays BxB, White has NxQ. Black therefore needs to spend his next move taking his queen out of danger, and White’s fork works after all. The general lesson: be mindful of the defensive work your pieces are doing before you send them off to attack.