Swordfish is the three-row version of X-Wing. The logic is identical — a digit is confined to specific rows and columns, making it eliminable everywhere else in those columns. Harder to spot, more powerful when you find it.
Find a digit that appears as a candidate in at most three cells in each of three different rows. If the candidate cells across all three rows are confined to the same three columns — you have a Swordfish.
Eliminate that digit from all other cells in those three columns.
In the solution, digit 6 must appear exactly once in each of the three rows. The three valid placements can only occupy cells in columns 2, 5, and 8. Each of those three columns will receive exactly one instance of 6 from these three rows. So 6 cannot appear anywhere else in those columns.
This is the same logic as X-Wing, extended from a 2×2 rectangle to a 3×3 grid of candidate cells.
The pattern works identically in columns. Find three columns where a digit is confined to at most three candidates each, all in the same three rows. Eliminate the digit from all other cells in those rows.
Most Sudoku solvers encounter Swordfish in expert-rated puzzles. If you've already applied X-Wing and the puzzle is still stuck, Swordfish on the same digit is the natural next step.
These three techniques form a family, differing only in scale. X-Wing uses 2 rows and 2 columns. Swordfish uses 3 and 3. Jellyfish uses 4 and 4. Each is rarer and harder to spot than the last. In practice, if a puzzle needs Jellyfish, you'd usually reach for a solver before finding it manually.