Chess Opening Theory/1. e4/1...e5/2. Nf3/2...d6/3. d4/3...Nd7/4. Bc4/4...Ngf6/5. dxe5/5...Nxe5/6. Nxe5/6...dxe5/7. Bxf7/7...Kxf7/8. Qxd8/8...Bb4

8...Bb4+
This is a forced move to regain as much material as possible. If White plays Nc3 or Nd2, or foolishly moves the king, he loses a piece after Rxd8. If c3 or Bd2, then after Rxd8 and a capture on b4, white would be better because of Black's weak e-pawn, but not a pawn up. So, the best move is 9.Qd2. After Bxd2+, Nxd2 guards the e-pawn, thus remaining a pawn up to the good.