Where does Black have the makings of a discovered attack? On the fourth rank. This pattern is easy to overlook during a game because of the pawn that lies between the Black queen and bishop. It's still important to see the kernel; teach your eyes to hop over pieces or pawns in the way of the pattern, since it may be possible to get rid of those obstructions, as is our challenge here. Black has a target in White’s queen on a4, and he has a check for the bishop in BxN+. All the ingredients are in place save one: the pawn on c4 must be removed. The usual way to clear an enemy pawn from a line you need is to capture something it protects, forcing it to move to recapture. Black does this with Nxd5. White can’t recapture with his c3 knight because it’s pinned; and if he plays c4xN, the way becomes clear for BxN+, discovering the fatal attack QxQ. Of course White should prefer just to forfeit the pawn.
There remains one complexity to consider: White could reply to Nxd5 with QxB. (Don’t automatically assume that a capture will provoke a recapture; it might provoke a fresh capture by your opponent elsewhere.) Of course this would leave White’s queen open to capture by the knight now on d5, but it also would unpin the knight on c3 and thus ready it to capture Black’s queen next move. So does White’s QxB ruin Black’s plans? Not quite. Imagine it playing out: 1. …Nxd5; 2. QxB, NxQ; 3. NxQ. When considering a little sequence like this that would reposition either or both of your knights, pause to consider whether there might be a kicker at the end in the form of a knight fork. Here the exchanges just described would leave a Black knight on b4—and able to drop next move to c2, where it forks White’s king and rook.