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...Bd7/10. Bxd7

10...Nxd7 preserves castling rights for Black. Thanks to the precarious position of the a7-knight, it doesn't matter that it unprotects e4. The alternative 10...Kxd7? would allow the exchange of queens with 11.Qb5+ and the knight is saved.