Diagram

*Position after 39... a4*

The idea of this sudden break is 40 bxa4 Rb1 41 Qe3 Qxc4, and Black seems fine. A very strange variation is 40 bxa4 Rb1 41 Re1! Rxe1 42 Qxe1 Qxc4 43 Qa5 and then Black should play 43 ... Kg8 although 44 Qb5 looks ok for White.

From: "K Hills"

Subject: Isnt 39...a4 a mistake?
From: KB Hills
Surely after 40 ba Rb1 41 Qe3 Qxc4 42 Bxe4 is strong as Rb8 seems necessary to guard the back rank eg 42 ...Ra1 43 Bc2 and the d pawn cannot be taken (43 ... Qxd5 44 Qe7+ Kg8 45 Qe8+ so it seems While is now a pawn and can play d6 (ie the d pawn is mobile.
Mr. Hills,
I am sticking by my original analysis (in Shaked - deFirmian, game 1) that 39 ... a4 was a good move. After 40 bxa4, 40 ... Nxd5 looked tremendous but actually White should come out on top after that. My suggestion 40 ... Rb1 would have been fine because after 41 Qe3 (better would be 41 Re1 although then 41 ... Rb4 or 41 ... Rxe1 42 Qxe1 Qxc4 are acceptable) Qxc4, Black is better as your 42 Bxe4 loses to 42 ... Rb3. See also Mr. Phipps' question on Black's move 40.

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