Black’s king is immobilized; his h6 knight must maintain its guard against Nxf7#; and moving his pawn would permit Rh7# (or Nxg6#). That means that only the e4 knight is free to move safely. With 1. Qb1! white immobilizes that piece as well, as moving it would now permit 2. g7#, since the queen now guards the king’s only escape square. So Black is out of safe moves, and must invite a mate.
