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.
| 5 | 3 | 8 | 9 | 2 | 1 | 6 | 4 | 7 |
| 9 | 7 | 1 | 6 | 4 | 3 | 2 | 8 | 5 |
| 4 | 6 | 2 | 8 | 5 | 7 | 1 | 3 | 9 |
| 2 | 8 | 9 | 124 | 356 | 148 | 5 | 7 | 3 |
| 6 | 5 | 3 | 279 | 389 | 246 | 4 | 1 | 8 |
| 7 | 1 | 4 | 156 | 289 | 368 | 9 | 2 | 6 |
| 1 | 9 | 7 | 5 | 6 | 2 | 8 | 3 | 4 |
| 3 | 4 | 6 | 7 | 1 | 8 | 5 | 9 | 2 |
| 8 | 2 | 5 | 4 | 3 | 9 | 7 | 6 | 1 |
In Box 1, digit 9: R1C1={2,9}, R1C2={3,5}, R2C1={4,6}, ... → 9 is only a candidate in R1C1 → R1C1 = 9.
This 9×9 puzzle is solver-verified to require this technique on its solution path.