After the usual 9. Qd2 O-O 10. O-O-O, Black has in 10… a5 a fitting alternative to the main 10… Nbd7 (Line 500). In response to 10… a5, White usually chooses one of the following options: 11. a4, 11. Bb5 and 11. Kb1.
White permanently stops the a5-a4 advance with 11. a4, but that comes at a cost of making his King somewhat vulnerable. After 11. a4 Nc6 12. g4 Nb4 13. Kb1 Rc8 Black gets sufficient counterplay thanks to the d6-d5 motive.
By choosing 11. Bb5 White keeps an eye on the a4-square, but Black is often ready to sacrifice his a-pawn for the initiative by playing a5-a4.
[Diagram: Black to Move] White’s poor piece coordination together with the weakened position of his King provide Black with an opportunity to strike a final blow. How can Black develop a decisive attack?
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