Unique Rectangle
Level: Advanced
Avoids a dangerous rectangle pattern using the uniqueness principle.
The Unique Rectangle relies on Sudoku's rule that a valid puzzle has exactly one solution. If four cells spanning two boxes hold only the same two candidates {X,Y}, leaving them as they are would create a contradiction with two possible solutions.
To avoid this deadly pattern, one of the four cells with an extra candidate must be confirmed as that candidate, or a specific candidate must be eliminated.
Because it relies on solution uniqueness, it's a distinctive advanced technique that only applies to valid puzzles.