A mysterious move, played by several strong grandmasters, including the ultimate boss, Magnus Carlsen. The idea isn't simple to understand. I suppose White is anticipating the move a6, but I also don't know why it matters if Black plays this move anyway or what the big advantage is in preventing it. In any case, this is yet another of the many attempts to surprise Berlin Defense players.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1b5  3... g8f6  4. d2d3  4... f8c5 