comment: The best way to fight for an advantage. It's useful for White to leave the pawns on the a-file.
context: 1. d2d4  1... d7d5  2. c2c4  2... d5c4  3. g1f3  3... a7a6  4. e2e3  4... b7b5 A much riskier line than the traditional ways of playing the Queen's Gambit Accepted with Nf6-e6-c5. Black's idea is not to end up with an extra pawn, because that is impossible, but to change the pawn structure on the queenside. 5. a2a4 Necessary to recover the pawn. 5... c8b7  6. b2b3  6... g8f6 