Suppose you spot a potential fork but find that the forking square is defended and there is no way to pin the defender, capture it, or capture anything it protects. It still isn’t time to give up; instead, imagine going ahead with the fork and letting your knight be captured. Picture the board as it then will look: without your knight, and with your opponent’s guard moved from its current square onto the forking square. What then would be possible? Consider the consequences of any checks you then would be able to inflict; consider whether the stage has been set for another fork; consider whether any pieces that used to be protected now are loose; consider whether any open lines have been created that would allow you to pin one piece to another. Indeed, sometimes playing fork and letting it fail is more powerful than "perfecting" it by removing the guard of the forking square. And sometimes you thus will find that an implausible-looking fork works after all because the cost to your opponent of taking your knight is too high. All this will be made clear with some examples.
In the position to the left White has the makings of a fork with Ne7+, but e7 is protected by the Black queen. The queen cannot be captured or threatened, and White can’t attack anything else the queen protects. So: what if White goes ahead with the fork and Black plays QxN? What will the board then look like? Two things will have changed: the White knight will be off the board and the Black queen will have moved from e6 to e7. Both changes are significant. The removal of the knight creates an open line on the c-file, allowing the White queen to attack the rook at c8; and the movement of the Black queen removes the rook’s protection, leaving it loose. So White then plays QxR+ and takes a rook for a knight, winning the exchange.