You want to notice pressure of the sort that Black's b6 bishop applies every time it occurs: it cuts off the dark squares in front of White’s king, which is walled in on the other side by its own pieces. Pressure added on the diagonal from h3 would mate; and to open the diagonal for that purpose we can make a threat or capture on f3 that forces the g2 pawn to capture. To apply the logic here: White guards the pawn on f3 not just with his pawn on g2 but also with his bishop on d5; yet Black also has two attackers trained on the square, and so can force open g2 thusly: 1. …Rxf3+, 2. BxR, RxB+; 3. g2xR, Bh3#.