Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6/6. Qe2/6...d5

Aron Nimzowitsch would have been proud of the move 7. Nxf6, a "liquidation with gain of tempo", in other words Black doesn't have time to make a normal developing move if you capture her knight with check. Plus, she's going to have to recapture with the g-pawn to protect e5, and then you can point and laugh at the hole in her kingside pawn cover (and hope that her brilliantly supported centre is not as solid as it looks).

White has made a few attempts to avoid the exchange on f6:
 * 7. Ng3 is possibly an underrated move. It makes more sense than the equivalent line 5...d5 6.Ng3, because the queen on e2 makes it impossible for Black to save her e-pawn. Having said that, Black was prepared to sacrifice that pawn anyway had White played 6.Nxf6 Qxf6 7.Qe2, so now she just sacrifices it differently.
 * 7. Nc3 just invites Black's pawns further forward.
 * 7. Neg5 has the idea that 7...e4 saving the pawn can be met by 8.Ne5 and White has two knights looking at f7. But again, Black doesn't necessarily want to save the pawn. 7...Bd6 8.Nxe5 O-O scores very well.
 * We should consider 7. Nxe5 too, because at first glance it looks like White is doing better than the equivalent 5...d5 6.Nxe5 line in having already gotten the move Qe2 in while Black has blocked her own queen from accessing g5. But it turns out that that f6-knight is the difference-maker, because it means that after 7...dxe4 8.Nxc6 Black can simply capture the knight, sacrificing the exchange and a second pawn for an awesome amount of activity, and White won't have the bucket of cold water that is Qh5+ in response.