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?