Preventing e7-e5.
1. g1f3  1... d7d5  2. e2e3  2... g8f6  3. c2c4  3... c7c6  4. b1c3  4... a7a6 This system in which Black plays the pawns to c6 and a6 is called the Chebanenko. It was one of the favorite defenses of GM Morozevich, one of the most original chess players in history. Black breaks the normal rules of the opening and prepares to advance the pawn to b5 before completing development. 5. d2d4  5... b7b5  6. b2b3 This move is the most popular, protecting the pawn on c4. The c4-c5 advance, gaining space, is also critical. 6... c8g4  7. a2a4 [%c_effect a4;square;a4;type;Interesting;persistent;true] Now Black needs to decide whether to capture the pawn on c4, opening the b-file, or close in the queen with b5-b4. There is no consensus on this and both moves have already been played in grandmaster practice. 7... b5c4 The machine's first choice. 8. b3c4  8... b8d7 