| notyetagm: Gelfand vs Wang Yue, 2008 White has just played 51 b2-b3?
Black to play: 51 ... ?
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<51...a3. An interesting alternative was 51...Bd1! and now 52.bxa4 (52.Bxd1 a3 wins) 52...Bxc2 53.axb5 cxb5 54.Kxb5 Kd6 and it is game over for White.> So Wang Yue (Black) played 51 ... a4-a3, creating a dangerous outside passed a-pawn. But instead he could have won on the spot(!) with 51 ... ♗f3-d1!. (VAR) Position after 51 ... ♗f3-d1! 
click for larger viewThe point is given in the principal variation (PV) of 52 ♗c2x♗d1 a4-a3. (VAR) Position after 52 ♗c2x♗d1 a4-a3
click for larger viewNow the White b3-pawn <SELF-BLOCKS> the b3-square, preventing the White d1-bishop from occupying this square and <DEFENDING> the a2-square against the advance of the Black a3-passer, the a2-square being the last chance for the light-squared White d1-bishop to stop the Black a3-pawn from promoting as the a2-square is the final light square in the passed pawn's path. The other variation after the instantly winning move 51 ... ♗f3-d1! consists of White capturing the dangerous Black a4-pawn but losing his White c2-bishop in the process, resulting in an obviously lost endgame. (VAR) Position after 52 b3xa4 ♗d1x♗c2 53 a4xb5 c6xb5 54 ♔c5xb5 ♔c7-d6
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click for larger viewSo Wang Yue still won after playing 51 ... a4-a3 but 51 ... ♗f3-d1! would have forced instant resignation. | 
Kibitzing
