The crucial square to focus on this time is g2. Black has a knight already attacking it, and his queen is aimed the same way; if he could get his queen onto g2, it would be mate. The queen’s path to g2 is blocked by his bishop. He could try attacking g2 by first moving his queen to c6, but this gives White a move he can use to defend g2 with Qg3. So focus instead on the g-file, where Black has the kernel of a discovery. If the bishop were gone the queen would threaten to mate, so Black looks for mischief he might make with his bishop—or, better, some way it might help with the queen's landing on g2. He finds no checks or captures for the bishop but identifies a move that nevertheless is effective: Be3, which attacks the bishop on f2 and pins it to White’s king; it also cuts off the path of the White queen along the third rank toward any possible defense of the g2 square. White is doomed:
(a) If he captures Black’s bishop with his own, his queen remains blocked; Black mates with Qxg2.
(b) If White instead uses his queen to take Black’s bishop, he likewise is mated immediately by Qxg2.
(c) If White advances his g-pawn to g3, Black plays Qxg3+ and mates a move later.
(d) If White advances his g-pawn to g4, he effectively blocks the descent of Black’s queen down the g-file, but Black plays Qc6 and again mates soon on g2; for the third rank still is impassable to White’s queen.