comment: The ultimate prophylactic move. White takes the knight out before the thematic e7-e5, which is fundamentally Black's plan after a7-a6. The move is still possible, but it's a bit discouraging to play it when your opponent has already taken the knight out of d4.
context: 1. e2e4  1... c7c5  2. g1f3  2... d7d6  3. d2d4  3... c5d4  4. f3d4  4... g8f6  5. b1c3  5... a7a6 