Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6/6. Nxf6/6...Qxf6/7. Qe2/7...Be7/8. Bxc6/8...dxc6

9. Nxe5 is the main line. White needs to capture the pawn now otherwise Black will play ...Bd6 and ...Bg4 and the window will be shut.

Why is it not a good idea to capture with 9. Qxe5, since White will be a pawn up with the queens off? Because Black doesn't have to obligingly exchange queens on e5. Instead she can play 9...Bg4 to exploit the brittle knight on f3, which won't have any good squares after 10.Qxf6 Bxf6, and if White has to play gxf3 her kingside pawn majority will be nobbled. (10.Qxc7? is suicide with White so under-developed. The simple 10...O-O! gives Black an irresistible attack.)