I was surprised to find out that Keymer had played this move before. It's beyond my very basic knowledge of this position and all I can say is that he is the only GM to try it and that it's impossible to understand Black's idea (considering White's next move) without very deep study of the position.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1c4  3... g8f6  4. d2d3  4... f8c5  5. e1g1  5... a7a6  6. c2c3  6... c5a7  7. a2a4  7... d7d6  8. f1e1  8... e8g8  9. h2h3 Italian move-orders are always full of subtleties. In any case, we have arrived at a very normal position. Black played the traditional a6-Ba7, White the no-less-common c3, a4, Re1, h3, all very logical moves. But now we're in for something very creative.