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

White now has the simple plan of establishing a knight on e5 guarded by a pawn on d4 (which isn't going to get taken en passant because it'll go there with a discovered attack on the Black queen). The two possible ways of carrying out this plan of Ne5 and d4 are (you guessed it): The immediate 10.Ne5+ means that the Black queen will definitely go to h4 after 11.d4, whereas 10.d4 gives it the option of retreating to d6 instead. If she declines that option and goes to h4 anyway, the two lines merge into one.
 * 10. Ne5+ followed by 11.d4, and
 * 10. d4 followed by 11.Ne5+.

White still has the Nxa7+ option. What's different about 10. Nxa7+ compared to 9.Nxa7+? On balance, those differences are in Black's favour, so if Nxa7+ is going to be played it's best on move 9 rather than 10.
 * Black no longer has the option of declining the f4-pawn after 9.f4 Qxf4 10.Nxa7+, as she does after 9.Nxa7+ Bd7 10.Bxd7+ Nxd7 11.f4.
 * But Black now has the additional option of 10...Bd7 11.Bxd7+ Kxd7. Recapturing with the king would be a bad idea after 9.Nxa7+ Bd7 10.Bxd7+ because White could trade queens with 11.Qb5+. With f4 and ...Qxf4 inserted, that's not a problem.