Where is the idea this time? You have a bishop trained on g7. One important effect of such a resource is that you can create a mate threat by adding an attack by the queen against the same square, as with Qg3; so the key question is whether you can make another threat with that move at the same time. You see that from g3 your queen would be aimed at the rook on b8. The position thus has the makings of a classic queen fork, but there are a few problems in the way of its execution. The rook is guarded by the knight on d7; the queen's path to the rook would be blocked by White's own rook on f4; and the forking square (g3) is guarded by the knight on e4. Yet none of these difficulties need detain you for long. 1. BxNd7, RxB is a simple exchange that leaves the Black rook loose; 2. RxN, QxR loosens the forking square and opens the needed line; 3. Qg3 calls for Black to reply Bf8 to stop the mate threat; and then 4. QxR nets a piece. (4. …Qxg2 gets back a pawn for Black.)
Now consider a few “why not” questions:
(a) Why not start with RxNe4 rather than BxNd7? The answer is that 1. RxN allows Black to recapture QxR and get his queen onto e4—a powerful posting—too early in the sequence. For then when you play 2. BxN, Black doesn’t recapture RxB; he plays Qxg2. Now the forking square you want no longer is available (Black’s queen guards it). You have won two minor pieces for a rook and a pawn, and if you aren’t careful Black is about to take another pawn with Rxe3. (Qxh3 also is available to him.) You can win back a pawn with 3. Rxd5, QxR; 4. Qg3 (at last you get to play the queen fork after all), f7-f6; 5. QxR+. But the overall sequence isn’t what you had in mind. Better to start capturing in the back of Black’s board with BxN, and save the invitation to Black’s queen until you’re ready to take strong hold of the initiative next move with a mate threat.
(b) That explanation also shows why, after White starts with 1. BxN, Black recaptures with his rook rather than his queen: he’s better off keeping his queen where it can get to the center of the board after White’s next move, RxN. And it shows why Black uses his queen to avenge that last White capture rather than his d5 pawn: the queen is powerfully positioned on e4 and in this case can grab the pawn on g2 after White does his damage.
(c) Finally, when White plays 3. Qg3, why should Black fend off the mate threat with Bf8? Why not, say, f7-f6 or g7-g6? Answer: those pawn moves would allow White to give check when he plays QxR+, thus forcing Black to waste a move saving his king when he should be playing Qxg2.