Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4. e3/4...b5/5. Nxb5/5...cxb5/6. Qf3/6...Nc6/7. Qxc6/7...Bd7/8. Qf3/8...e5/9. dxe5/9...Bb4

9...Bb4+
This move forces Ke2, since Kd1 Bg4+ and Bd2 Bxd2+ Kxd2 Bg4+ both win white's queen. Note how vulnerable white's king is, with the g4-d1 diagonal and d-file becoming open.