Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...d5/6. Nxe5/6...dxe4/7. Nxc6/7...Qg5/8. Qe2/8...Nf6/9. Nxa7
9...Bd7 is the main line. Black preserves castling rights and denies the a7-knight the chance of exchanging itself on c8, which would be the coward's way out. It looks as though the move overworks the f6-knight, but in fact there are tactical reasons why grabbing the e4-pawn doesn't lead to any advantage.
9...Kd8 invites 10.Nxc8 and it looks like White has no problems, but Black has a glimmer of hope based on the fact that g2 is still hanging. German correspondence player Peter Leisebein is the main champion of this line.
9...c6? 10.Nxc6! means the c8-bishop will have to go to d7 anyway, but with one fewer Black pawn on the board.