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. f4/9...Qxf4/10. Ne5/10...c6/11. d4/11...Qh4/12. g3/12...Qh3

The b5-bishop is finally forced to move, and 13. Bc4 threatens to revoke Black's castling privileges with a check on f7.

13.Nxc6 would be refuted by the same ...a6 and ...Bd7 trick, pinning and winning the knight, that we saw earlier.