|notyetagm: Position after 31 ♖b1-b2
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The position after 31 ♖b1-b2 is a perfect example of what I call <PINNED + LOOSE = ZUGWANG>.
Because the White b2-rook is on an open line on the 2nd rank, the White b2-rook is threatening ♖b2x♘c2 *and* ♖b2x♖d2.
This is the <BALEFUL INFLUENCE> property of line pieces, that they threaten <EVERY SINGLE SQUARE (ESS)> on their lines.
To meet the threat of ♖b2x♘c2, Black is <DEFENDING> the c2-knight,square with his Black d2-rook from the only square d2. So if the Black d2-rook moves, it puts the c2-knight <EN PRISE>.
And likewise, to meet the threat of ♖b2x♖d2, Black is <BLOCKING> the line from b2 to d2 @ c2, again from the only square. So if the Black c2-knight moves, it puts the Black d2-rook <EN PRISE>, as the line b2-d2 will no longer be <BLOCKED> at c2.
*So* if the Black c2-knight cannot move because it is <PINNED>, and the Black d2-rook cannot move because it must <DEFEND> the c2-square, <<<neither the Black c2-knight nor the Black d2-rook can move!>>>.
Hence the <ZUGZWANG> part. The only piece that Black can move is his Black e5-king, and that does *nothing* about the upcoming threat of 32 ♘d5-b4/e3, <REINFORCING THE PIN> on the <PINNED> Black c2-knight.