Tier 4 · first asked for by Diabolical
Remote Pairs
A chain of identical two-candidate cells alternates, so its ends between them cover both digits.
Also called Remote Pairs.
Find a run of cells all holding the same pair {X,Y}, each seeing the next. The values must alternate along the chain: if the first is X the second is Y, the third X, and so on.
Cells an even number of steps apart therefore match, and cells an odd number apart are opposites. When the ends of an odd-length run are seen by some other cell, that cell is looking at both X and Y, and loses them both.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. Remote Pairs on (27): the chain R4C7, R6C9, R8C7, R8C2 alternates, so cells seeing both ends lose both digits.
Builds on Naked Pair.