A mating pattern comes into view. The bishop on f2 seals off g1, and Black’s knight seals off h2; thus the dark squares around White’s king all are accounted for, so an attack on the long diagonal would have lethal potential. The g2 pawn can be pulled off that diagonal with a simple queen sacrifice on h3. If Black’s light-squared bishop were available, the idea would be complete. Unfortunately it’s way back on c8. But then you see that the bishop might be moved without a loss of time because it will unmask a discovered attack against White’s queen. Thus Black plays Bf5. This move puts White into deep trouble. His natural move—indeed, his only move that does not lose his queen—is QxR. But then Black mates in two strokes: Qxh3+, requiring White to reply g2xQ; and then Bxe4#. This was how Blackburne in fact ended the game. White's better reply to Bf5 would be d2-d3 or Qxh7+. These moves lose White’s queen but forestall mate.