Consider White's checks. He has several with his queen—e.g., Qc8, Qf7, or Qb5—but none of them force the king to move; the best is Qb5, which at least doesn’t lose the queen, but Black meets it with Qd7. So now White looks for checks with other pieces and finds Ba4+. This time Black can’t afford to play Qd7 because it forfeits his queen. He has to move his king to d8 or f8. Either move puts his king and queen on the same dark-squared diagonal. Can White take advantage with his dark-squared bishop? If Black plays Kd8 White has Bg5, pinning the queen with support from the knight on f3. If Black plays Kf8, White can play that same bishop to the other side of the board with Ba3, again pinning the queen (this time with backup from the rook on a1). So no matter how Black replies to Ba4+, his queen gets pinned by a White bishop a move later.