Now we're going to consider a few positions where the elements of a fork are brought into being by a judicious use of mating threats. These studies will be pretty hard, at least by comparison to those we've seen already; peruse them for the ideas involved. The whole question of the tactical use of mating threats will be explored in more detail, and in a more step-by-step fashion, in the next chapter (on queen forks).
Prospects for a knight fork for Black might seem remote in the position to the left, but it turns out to be easily accomplished. Begin by thinking about the enemy king and any pressure you can put on it. Ng4 is interesting because it seals off f2 and (more to the point) h2, leaving the White king trapped on the back rank just as it would be if there were a wall of pawns in front of it. You look for the follow up and see that Black would then be ready to mate with Ra1+ (White would have the useless interposition Rd1, to which Black replies RxR#). Now of course after Black plays Ng4 White has a move he can use to fend off the coming Ra1+; he plays Rd1, thus preparing to meet Ra1 with RxR. What now for Black? Anytime your knight is in the picture and enemy pieces have moved, consider forking possibilities. From g4 the knight would be able to jump to f2 with check and take the rook for free next move.
The trick to the position is to carefully look not only for any checks but also for any mating threats you can create—not necessarily because you expect them to lead to mate, but because you know they can create pressures that have tactical payoffs. A search for mating threats includes consideration of any move like Ng4 that, while not giving check, traps the enemy king in a tight space. Then you figure out what your opponent would have to do to defuse the threat, and ask whether any forks or other tactical moves would be possible on the board as it then would look.