comment: Nakamura has played this move a few times before in blitz games. It has pros and cons: White gives up an important bishop and the dark squares are somewhat weakened as a result of the pawn on b3. On the other hand, Black has to play with doubled pawns. It's a move that can only be recommended in rapid time control games.
context: 1. b2b3  1... c7c5  2. c1b2  2... b8c6  3. g1f3  3... g8f6  4. e2e3  4... g7g6 