Two cells in a unit share exactly two candidates; those digits are eliminated from all other cells in the unit.
When exactly two cells in a unit (row, column, or box) share exactly the same two candidates (e.g. {3,7} and {3,7}), those two digits must occupy those two cells. Therefore 3 and 7 can be eliminated from all other cells in that unit.
This relies on the "locking" principle: two values occupy two cells.
| 3 | 9 | 5 | 8 | 7 | 4 | 6 | 2 | 1 |
| 1 | 2 | 4 | 3 | 5 | 6 | 7 | 8 | 9 |
| 6 | 7 | 8 | 1 | 2 | 9 | 3 | 4 | 5 |
| 2 | 4 | 19 | 5 | 6 | 3 | 8 | 19 | 7 |
| 5 | 3 | 6 | 79 | 8 | 17 | 2 | 19 | 4 |
79 | 8 | 179 | 249 | 49 | 12 | 5 | 3 | 6 |
| 4 | 1 | 27 | 267 | 3 | 5 | 9 | 67 | 8 |
| 8 | 5 | 279 | 24679 | 49 | 27 | 1 | 67 | 3 |
79 | 6 | 3 | 79 | 1 | 8 | 4 | 5 | 2 |
This 9×9 puzzle is solver-verified to require this technique on its solution path.