How it works
A Swordfish is similar to an X-Wing but applies when all candidates for a digit in 3 rows are located in only 3 columns.
Since it’s impossible to have more than one instance of the digit in each column, the candidates for the digit must be located in these rows, so all the candidates in the remaining rows can be eliminated.
However, a Swordfish is never this neat. Not all the columns need to have a candidate for the digit.
It is sufficient if only 2 of the 3 columns in each row have a candidate, however, they also must be located so each column also have candidates in at least 2 rows.
This diagram shows a minimal Swordfish.
It is also possible to have 2 of the 3 defining rows in the same chute.
As with the X-Wing, the Swordfish can be applied using columns instead of rows.