A Finned Swordfish is an almost-swordfish with fin candidates. Same idea as Finned X-Wing, on a three-line fish: fish OR fin — only cover-cell eliminations.
Before this: Learn Swordfish and Finned X-Wing first.
Why it works
Either the clean swordfish is true (and clears a set of cells), or a fin is true (and clears a smaller overlapping set). Eliminations that survive both stories are valid. Fins shrink what you may erase.
Demonstration
Find an almost-swordfish with fin candidates in one box.
The green cells are a Finned Swordfish on 3. Either the fish holds, or the fin is true.
Only cells attacked in both stories — the fin’s cover cells — are safe to clear.
Eliminate 3 from the fin’s cover cells (red).
Example 1
Look for a swordfish shape with a fin.
This Finned Swordfish is on 2.
Remove 2 where the fin covers (red). Fish OR fin — only shared victims count.
Example 2
Look for a swordfish shape with a fin.
This Finned Swordfish is on 4.
Remove 4 where the fin covers (red). Fish OR fin — only shared victims count.