Turbot Fish

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A Turbot Fish is a same-digit two strong link chain. One strong link comes from a conjugate pair in a box, and the other comes from a conjugate pair in a row or a column. Unlike a Skyscraper or a Two-String Kite, the two strong links do not have a very obvious geometric relationship.

For a given candidate digit, if you can find one conjugate-pair strong link in a box and another in a row or a column, and choose one node from each so that those two chosen nodes lie in the same column or row and therefore form a weak link, then the pattern is a Turbot Fish. The two unchosen nodes are the endpoints of the chain. Any candidate that forms weak links with both endpoints can be eliminated.

For example, look at digit $3$ in the board below. There is one conjugate pair in row $3$: $3$ appears only in $R3C2$ and $R3C9$. There is another conjugate pair in box $6$: $3$ appears only in $R8C3$ and $R9C2$. These form two strong links. At the same time, $R3C2$ and $R9C2$ lie in the same column, so they form a weak link. This gives a two-strong-link chain with endpoints $R3C9$ and $R8C3$. By alternating-inference-chain logic, one of those endpoints must be true. Candidate $3$ in $R8C9$ forms weak links with both endpoints, so it can be eliminated.

The characteristic feature of a Turbot Fish is the conjugate pair cramped inside a box, which resembles the two close-set eyes of a flatfish. Here are some other forms of the pattern.