Played after some hesitation. The knight is  protecting the c4-pawn and is ready to jump on b3 at the right moment.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g1f3  3... d7d5  4. b1c3  4... d5c4  5. e2e4  5... b7b5 A very fresh line which was tried on a couple of occasions by Caruana. Apparently Nakamura liked  the idea and decided to give it a try. 6. e4e5  6... f6d5  7. c3b5  7... d5b6  8. a2a3  8... b8c6  9. c1e3 