Untie the knot in the center by asking what attacks what and what defends what. Going piece by piece, the answer turns out not to be complicated. White attacks Black’s e4 knight twice (never overlook a fianchettoed bishop); the knight also is defended twice, by Black’s queen and bishop. If one of the Black guards could be removed, the knight would be takeable. White has no way to capture Black’s bishop or queen. Can he threaten them? Consider pawn threats. White has 1. g3-g4. The obvious response is to move the bishop to another square from which it still can protect the knight, so Black plays Bg6. Except now White can capture the guard, with 2. NxB. After Black recaptures 2. ...RxN, his knight on e4 is attacked twice and defended just once. White takes it next move.
Well, but how does White take the e4 knight—with his bishop or his queen? If he uses his queen, it goes 3. QxN, QxQ; 4. BxQ, Rxg4+, 5. Bg2 and White has won a piece for a pawn. But if White first uses his bishop instead of his queen, things play out a little differently: 3. BxN, Rxg4+. (Notice the point: when he started by moving his bishop off of the g-file, White turned Black’s capture Rxg4 into a check that seizes the initiative before White has finished his sequence.) White plays 4. Kf1 (not Bg2, resulting in QxB#), and now Black attacks the White bishop twice against one defender. He plays 4. …RxB; 5. Qxa5. Each side has won two minor pieces and a pawn—a wash. So White should play 3. QxB, not 3. BxB.
Finally, what about the possibility that Black might reply to White’s initial threat with a counterattack? After 1. g3-g4, Black could reply b7-b5—and now each side has a pawn threat against a minor piece. Not a problem for White: he goes ahead with the exchanges, 2. g4xB, b5xN, and now his mission is accomplished: the knight on e4 has only one guard, so White takes it with QxN.