White has one queen check: Qxg6. This also attacks the bishop on f5; but the bishop is protected, and can capture on g6 anyway. So the next question is whether White has any other pieces attacking the bishop that he might use to force an exchange which will leave g6 occupied by a more suitable target. The answer is 1. RxB, RxR. Now 2. Qxg6+ takes the rook left on f5 and so nets a piece and a pawn. (Black was required to use his rook to recapture, of course, because the g6 pawn was pinned.)