|notyetagm: Keres sure knows that <KNIGHTS NEED FORKING SQUARES>.|
Black To Play: 10 ... ?
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The Black b4-knight has a <FORKING SQUARE> on d3, from which it attacks the White e1-king. The White d2-queen is simply <OVERWORKED>, having to defend both the d3-forking square and the b2-target square. The White queen cannot defend both of these squares, which lie a knight's move apart (b2-d3).
Black (Keres) exploits the <OVERWORKED> nature of the White queen by playing the crushing 10 ... xb2!.
The point is that if White recaptures with 11 xb2, then the White queen is simultaneously <DEFLECTED> from the d3-forking square and <DECOYED> onto the b2-target square. Black then executes his knight fork with 11 ... x+ and 12 ... x, leaving him ahead by a whole piece with queens off the board.