notyetagm: White to play: 61 ?
click for larger viewHere the Black c6-rook must meet the threat of the White c7-pawn promoting on c8 by
<DEFENDING> the c8-promotion square, so the Black c6-rook cannot leave the c-file.
That is, the <OBLIGATION> of the Black c6-rook to meet the threat of c7-c8= +
<RESTRICTS> the <MOBILITY> of the Black c6-rook to the c-file, i.e., the Black c6-rook
does not control -ANY- squares along the 6th rank.
Now consider the White a6-pawn. Is it threatening to do anything? <<<Yes, it is!>>>.
The White a6-pawn is threatening to advance to the a7-square, in which case the
White a7- and c7-passers would simply overwhelm the Black c6-rook. How is Black meeting
this threat, the threat of a6-a7? Is Black <PINNING> the White a6-pawn? No. Is Black
<DEFENDING> the a7-square, like he is <DEFENDING> the c8-square to prevent the advance
of the White c7-pawn? No. <<<Black is meeting the threat of the advance of the White a6-pawn
-ONLY- by <BLOCKADING> it with his own Black a6-pawn!>>>. For the Black a7-pawn to
<BLOCKADE> the White a6-pawn to prevent its advance, the Black a7-pawn must stay on the a7-square.
Hence the <OBLIGATION> of the Black a7-pawn to meet the threat of a6-a7 <RESTRICTS> the
<MOBILITY> of the Black a7-pawn to the a7-blockading square, i.e., the Black a7-pawn -CANNOT- move at all. <<<Now, if the Black c6-rook cannot leave the c-file because it must <DEFEND> the c8-promotion square, and if the
Black a7-pawn must stay on the a7-square to <BLOCKADE> the White a6-pawn, then neither the Black c6-rook nor
Black a7-pawn <DEFENDS> the b6-square, right?>>> So what can we conclude about the number of defenders of the b6-square? There are -ZERO- defenders of the b6-square.
Why? Because both the Black a7-pawn and Black c6-rook have other threats to meet (<OBLIGATION RESTRICTS MOBILITY!>). Hence Volkov (White) played 61  d5-b6! and Hou Yifan resigned immediately. Position after 61 f5-b6! 1-0
click for larger viewThe White b6-knight has <GAINED CONTROL OF THE PROMOTION SQUARE> (c8-square) of the
White c7-passer. Black will then be forced to give up her rook the pawn. 61 d5-b6! is a lovely tactical shot the wins the game on the spot, based on the tactical point
that both the Black a7-pawn and c6-rook are <OVERWORKED> so that neither <DEFENDS> the b6-square. | 
sac: 53.Rxf5
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