Black has three pieces trained on the sector of the board where White’s king sits. Most important is the pin of the g2 pawn by Black’s bishop. It’s not the pin of the pawn per se that is critical; it's that the bishop has a pin on the g2 square and whatever happens to be sitting there. It may be possible to upgrade to a better target by taking the pawn and inviting a recapture. Here the only White piece that would be able to recapture if the pawn were taken is his queen, so the stakes are high enough to justify a queen sacrifice by Black. He starts with Qxg2+. QxQ is White’s forced reply—and now his queen is pinned. While White can play QxB, he can't move the queen off of its diagonal; so anything it appears to protect, it doesn’t. What the queen appears to protect—but no longer does—is the rook on e2, so Black takes it with RxR. Now White goes ahead with QxB, but after Black plays b7xQ he has won the exchange and a pawn.