This is the critical move.
1. d2d4  1... g8f6  2. g1f3  2... e7e6  3. c2c4  3... f8b4 The Bogo-Indian Defense is not very popular among the elite, but it's a great defense, especially for club-level players, as the plans are easier to understand than, for example, the Queen's Indian, one of the most positionally complex openings in chess. 4. b1d2  4... d7d5 This move has a dubious reputation, but Mamedyarov played it in the first round of the tournament and in the Olympiad as well, so it's clear he has his own opinion.