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
Now Black's attack against g2 is a real one. Defending g2 with moves like 9.0-0 or 9.Rg1 is too slow; Black will have time to execute her secondary threat of ...a6 and ...Bd7. The clever 9. f4 solves this problem by defending g2 with gain of tempo, while threatening to give the knight a beautiful square on e5 to retreat to, at the cost of restoring material equality (9...Qxf4 is the main reply).
9. Ne5+ c6 10.f4 as played by Gyula Sax is just the same moves in a different order.
But there is an alternative in the controversial 9. Nxa7+!? as played by Peter Svidler. It has a very positive score for White, but from a very small sample size. If you can't see what a brilliant move 9.Nxa7+ is, it's probably because
the Big 9.f4 industry has hired stooges to infiltrate our message boards and talk it down Magnus Carlsen can't see what a brilliant move 9.Nxa7+ is either.