Find the decisive blow (difficulty 1520)

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Black to move

Show solution
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  1. g8–f6 key move
  2. g1–f3
  3. b8–g3
  4. h1–g1
  5. g2–f3
  6. h2–g3
  7. f3–b7
  8. c4–b5

What makes it

A single quiet move decides it. The key move is the knight's. It calculates 8 plies deep.