The "Swordfish" technique is used to solve hard Sudoku with many empty (unfilled) cells. The technique helps to exclude hypothetically suitable candidate numbers and to reduce the scatter in the selection. In mechanics, "Swordfish" is similar to "X-Wing", but uses 3 columns and 3 rows instead of two. That is, to use the technique, you must go to a certain level of abstraction and find three rows or three columns that each have two occurrences of candidate numbers.
Let's look at an example of using the Swordfish technique. First, we need to find the so-called "fish digit" and the "base". In the example, the "fish digit" is a 6, and the lines 1, 6, and 9 are the "base" (highlighted by the green guide lines). There are only 2 occurrences of the six in each of these rows. The cells containing the six entries are lined up exactly in 3 columns (1, 2, and 9 - for clarity, they are also highlighted with a white and orange border). So all conditions are met - 3 rows, 3 columns. The "swordfish" construction is found!
Next, we can delete all notes in the three columns containing the number six (the second picture shows the candidates crossed out).
"Swordfish" is a Sudoku technique that requires very careful study of all rows and columns to solve. As a rule, the main problem for players is to find the necessary cells, the construction can be difficult to see in the mass of all the notes. At the same time, the technique itself is very effective for weeding out unnecessary candidates..