What is the idea of White playing 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. c4 ? Does he intend to get some good version of the main lines, while avoiding certain sidelines? It seems in fashion now with games like this one, but I am not sure I grasp the full concept - Ian Nepomniachtchi vs Sanan Sjugirov, 2016
In the main line 5.d4, black responds with 5....d5 in order to occupy the centre and to continue his development. The most popular line continues with: 6.Bd3 Be7 7.0-0 Nc6! 8.c4 Nb4! 9.Be2 0-0 (e.g. Nakamura-Akobian). Important to note: after 7....0-0 white has good chances to obtain an advantage after 8.c4.
With 5.c4 white wants to discourage black to play ...d5. After 5....d5 white seems to obtain an advantage with 6.Nc3. 6....Nxc3 is very dubious: 7.dxc3 Be6 8.Ng5 with a big advantage for white. Instead, black often plays 5....Be7. After 6.d4 0-0 7.Bd3 d5 8.0-0 white has successfully transposed to 5.d4 d5 6.Bd3 Be7 7.0-0 0-0 8.c4 (Nepomniachtchi-Wang Hao (blitz)).
However, black has better alternatives after 5.c4, for example:
- 5....Nc6 (Nisipeanu-Giri) 6.d4 d5
- 5....Be7 6.d4 Nc6 7.Bd3 Ng5
[StartPly "8"] [FEN ""] 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.c4 (5.d4 d5 6.Bd3 Be7 7.O-O Nc6 (7...O-O 8.c4) 8.c4 Nb4 9.Be2 O-O) 5...Be7 (5...d5 6.Nc3 Nxc3 7.dxc3 Be6 8.Ng5)(5...Nc6 6.d4 d5) 6.d4 Nc6 (6...O-O 7.Bd3 d5 8.O-O) 7.Bd3 Ng5
Just a tiny bit of history to explain the modern opening: It was Paul Morphy who started using the early ...Be7 and ...Nc6 which became the main line until the even more recent early Nc3 (without d4). However, Morphy had a penchant for ...f5 in many positions where it wasn't so wise, so 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 6. Bd3 Be7 7. O-O O-O 8. c4 f5 was his idea of securing the center. Today's players avoid that unless they're aiming for Pillsbury's idea of ...Bg4 and only then ...f5.
All this is tossed out the window by Kaufmann (is it Larry Kaufmann?) who avoids all that.