In the Norwegian Defense of the Ruy Lopez
[FEN ""] 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3 Na5 6. O-O (6. d4 exd4 7. Nxd4 c5? 8. Bd5!)
Neil MacDonald suggests
6. O-O as a response. Why isn't
6. d4 suggested in the book?
6... exd4 7. Nxd4 c5? does not work because of
8. Bd5! when White enjoys a comfortable lead in development. I ask what are the repercussions behind playing
6. d4 possibly followed by
7. O-O. Is this variation sharper?
The author suggested
6. O-O as a solid choice compared to the sharper
6. d4, but I don't see what's so sharp about it. White is simply challenging Black in the center. I guess it gives less information away about the central pawn structure. Is that the answer?
0-0gives minimal advantage to the White in all lines, while
d4offers equality. Briefly playing through the lines, I came to the same conclusion. Regarding the author's comment: It became prevalent today to play risk free lines with slight pull instead of sharp lines which can be improved with the engine. It is way easier to try and put pressure on your opponent from a quiet and slightly better position, instead of relying that your computer analyses sharp lines better than his.