The absurd-looking 1. Qb4! leaves Black with no options. If the bishop moves (or if the king moves to e3), then Ng4 is mate. If the king moves to g1, or if the black queen moves to e3, then Qxe1 is mate. If the black queen moves anywhere else along the a7-g1 diagonal then the white queen captures her, giving mate. And if she abandons that diagonal then White can sieze it, giving mate.