"Swordfish" technique.

The "Swordfish" technique is an advanced Sudoku strategy. It's usually applied in the hard levels of Sudoku puzzles to eliminate candidates. "Swordfish" is similar to X-wing but uses three sets of cells instead of two.

To understand better, let's take a look at the example.

In this puzzle 6 is our "fish digit" and rows 1, 6 and 9 are the base sets. The candidates for number 6 also align up perfectly in 3 columns. So, there are two options for number 6 to reside.

Either this way

Or this way

Either way, those 3 sets cover the aligned columns meaning 6 cannot appear twice there. Hence, we can safely eliminate 6 from all other notes in these columns.

Now you know how to apply "Swordfish" technique in Sudoku. It's very hard to spot but tremendously useful for your sudoku-solving arsenal.