Tier 5
X-Chain
A chain of alternating links in one digit, with strong links at both ends.
Also called X-Chain, Turbot Fish.
Follow a single digit through alternating strong and weak links. A strong link is a unit where the digit has only two homes: at least one is true. A weak link is any two cells sharing a unit: at most one is true.
Start and end on strong links and the chain proves that at least one of its two ends holds the digit. Anything that can see both ends therefore cannot, and loses the candidate.
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. X-Chain on 6: R1C9 → R1C7 → R9C7 → R9C8 → R6C8 forces R1C9 or R6C8 to be 6, so 6 leaves R4C9, R5C9.
Builds on Simple Colouring.