Sometimes a knight fork is available but only one of the forkable pieces is a good target; the other is a piece of equal or lesser value to your knight, and perhaps is guarded. The question then is whether the bad target can be made a good one through an exchange.
We start here with a case where at first glance there might seem to be no forking possibility for Black. The point to see is that Nc5 is a potential knight fork even though it won't work now because one of its targets is another knight (so Black's Nc5+ is simply met with NxN). Ask whether you can make your opponent replace the knight with something better by forcing an exchange. Here Black gets it done by taking the knight with his rook: 1. …RxN, BxR, and now Nc5+ wins the bishop and thus gains two pieces for the rook.