Hello and welcome to the swordfish technique course!
The swordfish technique follows the same principle as the w wing, with the difference of having three basic sets instead of two.
Let's look closely at the sudoku grid below.
The 6 is the fish figure and the lines R1, R6 and R9 are the base sets.
The candidates for the number 6 in rows R1, R6 and R9 line up perfectly in 3 columns C1, C2 and C9.
There are only two possible options.
Option number one, cells R1-C2, R9-C1 and R6-C9 contain the digit 6.
Option number two, cells R1-C9, R6-C1 and R9-C2 contain the number 6.
In any case, these 3 basic sets cover the aligned columns, which means that 6 cannot appear anywhere else in these columns.
So we can confidently remove candidate number 6 for the companion cells.