White’s inspection of the Black king’s lines turns up a pin of the pawn on g6. How to exploit this? By making aggressive use of one of the two squares the g6 pawn is supposed to protect: h5. Make a threat from there; deliver a check from there. White does it with Qh5+. Black is forced to move his king to the g-file. Whether he moves it to g7 or g8, White now plays Qxg6+, with the queen enjoying protection from the bishop that formerly imposed the pin (a common pattern we will see repeated). The king retreats to h8, and now White mates with Qh7#.