Does White have any loose pieces? Yes, the rook on d7. How many checks does Black have? Three: Qe2, Qd1, and Qb5 (examine the queen’s movements on every axis open to it—vertical, horizontal, and diagonal). You ask whether the queen can attack the rook from any of those squares, and see that it can do it from d1 or from b5; but of course at d1 the queen is unsafe, as it gets taken by the rook. So Qb5+ is the winning move.