Learn / Specialty reference
Sudoku technique · Specialty reference · reference
Templates / pattern overlay
Consider all placements of one digit that meet the row, column, and box constraints. Their overlaps and incompatibilities can force exclusions.
How to spot it
Think in terms of complete digit-placement patterns.
The common mistake
Template enumeration is implemented differently from local pattern recognition and can be expensive.
Learn this first
Candidate logic.
Further reading: hodoku.sourceforge.net