A digit that can only go in one cell within a unit must be placed there.
If a digit can only appear in one cell within a unit (row, column, or box), it must be placed there — even if that cell has other candidates. The digit has nowhere else to go in that unit.
Example: If 7 can only fit in R5C6 within Box 5, then R5C6 = 7.
19 | 5 | 19 | 2 | 3 | 7 | 4 | 18 | 6 |
12 | 3 | 4 | 6 | 8 | 9 | 15 | 15 | 7 |
| 6 | 7 | 8 | 1 | 4 | 5 | 2 | 3 | 9 |
19 | 2 | 6 | 8 | 5 | 4 | 19 | 7 | 13 |
| 3 | 4 | 5 | 9 | 17 | 2 | 6 | 18 | 18 |
179 | 8 | 19 | 3 | 17 | 6 | 159 | 2 | 14 |
| 4 | 1 | 3 | 5 | 9 | 8 | 7 | 6 | 2 |
| 5 | 6 | 7 | 4 | 2 | 3 | 189 | 189 | 18 |
| 8 | 9 | 2 | 7 | 6 | 1 | 3 | 4 | 5 |
This 9×9 puzzle is solver-verified to require this technique on its solution path.