How it works
Nishio sits at the edge of human technique. Purists consider it guessing; pragmatists use it when chains and uniqueness fail.
The procedure: pick a candidate in a bivalue cell. Tentatively place it. Follow every forced move (singles, pointing pairs, anything that does not require a new assumption). If the chain leads to a contradiction (two of the same digit in a unit, or a cell with no candidates), the original candidate was wrong and can be eliminated.
If the chain leads to a valid completion, you have solved the puzzle. If the chain stalls without conclusion, undo and try a different starting candidate.
When to look for it
Last resort. If forcing chains and coloring fail to crack an evil puzzle, Nishio on the most constrained cell often unblocks.
Tips for spotting the pattern
- Pick starting cells with rich consequences. Bivalue cells in tight units produce fast contradictions.
- Use coloured pencil marks (or digital ones) so trial placements are clearly distinguishable.
- Document the chain as you go. If you reach a contradiction, you need to know what to erase.
Common mistakes
- Forgetting to undo trial placements. Stale marks corrupt the rest of the solve.
- Calling it guessing. The elimination is logical: the contradiction proves the candidate impossible.
- Using it before simpler techniques. Most evil puzzles yield to coloring or forcing chains first.