Where does White have the kernel of a discovered attack? On the g-file. You look for a target for the rook once it is unmasked and find none. If only the king could be drawn onto the g-file, you think; and so you look for ways to make it happen. What checks are available? Qg8+. Black’s only legal reply is KxQ, moving his king onto the sensitive line. Now if White moves his bishop he gives check with his rook, so the question is what move to make with the bishop. It had better be good, since White sacrificed his queen to get here. A combined attack against the king is indicated; Be6++ checks the king with both bishop and rook. Black’s only move is to retreat the king to h8. Now comes mate: Rg8#, with the rook attacking the king and its only flight square, and getting cover from the bishop.