Box-Line Reduction
Intermediate Removes candidates Used in: Sudoku, Killer Sudoku
The mirror image of a pointing pair. A digit’s only candidates in a row (or column) all fall inside one box. The digit is then locked to that box, so it can be erased from the box’s other two lines.
How to spot it
Along a row or column, check whether a digit’s only candidates sit inside a single box.
What you can conclude
Delete that digit from the box’s other two lines.
Worked example
A row’s only 7s fall inside one box; 7 is then impossible in that box’s other two rows.