What White pieces are loose? The rook at e1 (as well as the queen, but focus on the rook for the usual reasons). Black has no safe way to check the White king and attack the rook at the same time, so he looks for a mating threat. Here as before the bishop is the answer: it attacks the b2 square (and pawn) adjacent to the king. Black would mate if his queen were to land on that square. If you followed the pattern of the previous problems, you might look for a way to put the queen on the same diagonal as the bishop—for example, by playing Qd4. But that doesn’t attack the rook, and anyway d4 is protected by White’s queen. So Black looks for other squares his queen could reach that would allow it to attack b2. He finds Qb4, which threatens mate by Qxb2 and also threatens, and wins, the loose rook at e1. The point: there is more than one way for the queen to attack a mating square already being hit by a bishop.