If a digit can go in only two or three cells in each of three rows, and all those cells lie within the same three columns, the three rows will put the digit in those three columns. So it can be removed from every other cell in them. It works the same with rows and columns swapped, and with four lines it's called a Jellyfish.
Swordfish is the X-Wing with one more row. A row doesn't need all three columns; between them, the three rows just mustn't use more than three.
It is rare, but when it appears it can clear a whole column at once.
A Swordfish
Where the pattern lives
The pattern
Candidates removed
Solved as a result
In columns 6, 8 and 9, the 5 fits only within rows 1, 2 and 6: R1C6, R2C6, R1C8, R2C8, R6C8, R2C9 and R6C9. Each of columns 6, 8 and 9 needs the 5 once and can only place it in rows 1, 2 and 6, so they put one in each of rows 1, 2 and 6. No other cell there can hold it: R6C1. That leaves R6C1 with only one candidate: 6.
Only the 5s are shown, so the pattern stands out.
This position comes from a puzzle generated on this site, after every single has been filled in. Boxes are numbered 1–9 from the top left, row by row; R5C4 means row 5, column 4.
How to spot it
1Pick a digit and look only at its candidates.
2Find rows where the digit has only two or three possible cells.
3Look for three such rows whose cells all fall within the same three columns.
4Remove the digit from every other cell in those three columns.
5Try the same with columns: three columns whose cells fall within three rows. Then clear those rows.
Why it works
Each of the three rows needs the digit exactly once, and can only place it in the three columns.
Three rows, three columns, three digits: every one of those columns gets its digit from these rows.
A column holds each digit once, so the rest of those columns can't have it.
Common mistakes
Counting a row whose cells stray into a fourth column. Every cell must be inside the same three columns.
Clearing the wrong lines: if you started from rows, you clear the columns.
Expecting every row to use all three columns. Two cells per row is fine, as long as the three rows together stay within three columns.
Questions
How is it different from an X-Wing?
An X-Wing uses two rows and two columns. A Swordfish uses three, and each row only needs two or three of the cells.
What is a Jellyfish?
The same pattern with four rows and four columns. It is rarer still; the hints here use it when nothing simpler works.
Why is it so rare?
Most of the time a simpler move, like a pair or a single-digit pattern, does the same job earlier.
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