AIC
An alternating inference chain proves one of its two ends is true.
Learn the AIC Sudoku technique (alternating inference chains): a chain of strong and weak links proves one end is true, so a candidate seeing both ends is cut.
How to use it
- Build an alternating chain. Link candidates by alternating strong links (if one is off the other is on) and weak links (they cannot both be on), starting and ending on a strong link. Links may switch digit through a cell.
- Read the two ends. A chain that starts and ends on a strong link proves at least one of its two end candidates is true — you do not know which, but one of them holds.
- Eliminate what sees both ends. Any candidate that is weakly linked to BOTH ends would force both ends off, which is impossible — so that candidate can be removed.