A Swordfish with an extra candidate cell, the fin. Eliminations are restricted to cells seeing both the Swordfish columns and the fin's box.
A Finned Swordfish is to a regular Swordfish what a Finned X-Wing is to a regular X-Wing. You have an almost-perfect Swordfish pattern (a digit confined to 2-3 cells in each of three rows, all within three columns) but one row has an extra candidate position (the fin) that breaks the clean pattern.
Despite the fin, useful eliminations remain. The logic considers two cases: either the fin cell doesn't contain the digit (and the Swordfish works normally), or it does (eliminating from the fin's peers). Cells eliminated in both cases are safe targets, specifically, cells in the Swordfish's columns that also see the fin cell.
Finned Swordfish is more common than regular Swordfish because the pattern requirements are looser. It appears in hard Extreme puzzles and is worth recognizing even if you don't actively scan for it.
Find three rows where a digit appears in 2-3 cells each, and those cells fall within three columns, except one row has an extra cell outside those columns. That extra cell is the fin.
If the fin is removed, you would have a standard Swordfish and could eliminate the digit from all other cells in those three columns. The fin restricts this: only cells in those columns that also share a box with the fin can be eliminated.
Apply the same two-case logic as Finned X-Wing: Case 1 (fin is off) = normal Swordfish eliminates from the columns. Case 2 (fin is on) = fin eliminates from its peers. The intersection of both cases gives you safe eliminations.
Example 1: Finned Swordfish
Scanning digit 1, columns 1, 5, and 7 almost form a Swordfish: their 1-candidates fall in rows 1, 7, and 8 — except column 7 carries one extra candidate at R3C7, the fin. The base cells are R7C1, R8C1 (col 1), R1C5, R7C5 (col 5), and R1C7, R8C7 (col 7).
Without the fin, this Swordfish would remove 1 from cover rows 1, 7 and 8 in every non-base column. The fin restricts the eliminations to cells that also see R3C7 — the only one is R1C9, which sits in cover row 1 and shares the fin's box.
Eliminate digit 1 from R1C9. Either the fin R3C7 is 1 (removing 1 from R1C9 in the same box) or it is not (and the clean Swordfish removes it) — either way, R1C9 cannot be 1.
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