X-Chain

Extend the turbot fish: strong link, weak link, strong link, weak link, and so on, always for the same digit, and always starting and ending with a strong link. If the first cell is false, the chain forces the last cell to be true. So at least one of the two ends is true.

Remove the digit from every cell that sees both ends. The chain must not visit the same cell twice.

Notation

X-Chain: 8 can be removed from r1c4. chain: 8 in r1c7 → 8 in r7c7 → 8 in r9c8 → 8 in r9c1 → 8 in r2c1 → 8 in r2c5.