1. d2d4  1... d7d5  2. c2c4  2... e7e6  3. g1f3  3... g8f6  4. b1c3  4... a7a6 [%c_effect a6;square;a6;type;Interesting;persistent;true] This is a popular variation these days. The idea is to threaten the capture of the c4-pawn, similar to what happens in the Semi-Slav Defense, which occurs after 4...c6. The difference is that, it seems, the move 5.Bg5 is worse for White in this case: Black can confidently take the c4-pawn.   So how should White defend the pawn $2 If they capture on d5 (the most popular choice), then Black clears the way for the bishop on c8. It's worth mentioning that the move a7-a6 is not a waste of time in this case, since the move has some use (for example, if White tries the famous minority attack).   If White plays 5.c5, as she does in the game, then Black can immediately break the structure with b7-b6.   In short: this move is better than it looks $1 5. c4c5  5... b7b6  6. c5b6  6... c7b6  7. c1f4 