Black’s two rook moves have created this situation. Now comes the payoff, starting with 3. …Ng4++. Since it is double check White cannot capture and must instead move his king to g1—its only square. Everything is in place for the denouement: Bh2#.

The most valuable thing to study in this position is the arrangement of bishops and knight in Black’s position. Neither the dark-squared bishop nor the knight are in the pattern for Blackburne’s mate, but they are in a position to reach that pattern; they can be scrambled into place in two moves if the White king is forced to cooperate (which is where Black's rook comes in). Go over the movements of the Black knight and bishop in this diagram (and the prior one) until the pattern flows visually in your mind’s eye.