Puzzles : Diagonals, Geometrical Motif

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         Black to move and win.
The winning move for black is,
1….Nf3+
forking both White Queen and King, forcing white to take the black knight with,
2.gxf3….
This leaves black to capture the e1-a5 diagonal by black bishop capturing the knight at c3 with
2….Bxc3.
This move pins the white Queen, forcing it to take the black bishop with black winning the white Queen and a strong winning advantage for Black.
(source: CT ART-3)