When a digit in a box is confined to one row or column, it can be eliminated from that row or column outside the box.
When a digit in a box can only appear in cells that all share the same row or column, that digit can be eliminated from all other cells in that row or column outside the box.
This uses the box-line interaction: since the digit must be in that row/column within the box, it cannot be in that row/column outside the box.
| 8 | 2 | 9 | 4 | 3 | 1 | 5 | 6 | 7 |
| 1 | 3 | 46 | 5 | 69 | 7 | 2 | 8 | 49 |
| 5 | 46 | 7 | 2 | 8 | 69 | 1 | 3 | 49 |
| 2 | 7 | 1 | 368 | 4 | 368 | 9 | 5 | 68 |
| 6 | 8 | 5 | 9 | 1 | 2 | 7 | 4 | 3 |
| 4 | 9 | 3 | 68 | 7 | 5 | 68 | 1 | 2 |
| 3 | 46 | 2 | 1 | 69 | 4689 | 68 | 7 | 5 |
| 7 | 5 | 8 | 36 | 2 | 36 | 4 | 9 | 1 |
| 9 | 1 | 46 | 7 | 5 | 468 | 3 | 2 | 68 |
This 9×9 puzzle is solver-verified to require this technique on its solution path.