Here the kernel is on the h-file; if the bishop moves, White’s queen gives check and Black’s king has to move. What to do with the bishop? It travels on the dark squares, and the only Black piece on a dark square is the queen, which the bishop can’t reach in two moves. But wait: think more carefully about the first part of the attack—the part against the king. Where would the king go? Its only escape would be to g7. If White could attack that square with his bishop, the king would have no place to go. So Bf8 is mate: a case of discovered checkmate. The important lesson is that when you think of where you might move the unmasking piece, don’t overlook pressure it can exert against squares, and especially against squares next to the enemy king.