If a digit can go in at most three cells in each of three rows, and all of them fall in the same three columns, the digit fills those columns from those rows. The rest of the three columns lose it.
On a Sudoku X board this works exactly as in Classic. The examples are from Sudoku X puzzles on this site.
Worked examples
A position from a real Sudoku X puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
Three rows, three columns
Look at three rows. Where does digit 6 appear?
Where it fits
ExtremeSwordfish is an Extreme technique: Extreme Sudoku X puzzles are the first to need it.
The Classic Swordfish lesson teaches it on a plain 9×9 board. The Sudoku X overview explains the rule and the levels.
Swordfish in the other styles: Jigsaw, Thermo, Windoku, Arrow.
Try it on an Extreme Sudoku X puzzle. Next Step points it out when it is the move.