If a digit can go in at most three cells in each of three rows, all in the same three columns, the rest of those columns lose it.
On an Arrow board this works exactly as in Classic. The examples are from Arrow puzzles on this site.
Worked examples
A position from a real Arrow puzzle on this site. Step through it: the board lights what each step talks about, and the red lines show what rules what out.
Three rows, three columns
Look at three rows. Where does digit 2 appear?
Where it fits
ExtremeSwordfish is an Extreme technique: Extreme Arrow puzzles are the first to need it.
The Classic Swordfish lesson teaches it on a plain 9×9 board. The Arrow overview explains the rule and the levels.
Swordfish in the other styles: Sudoku X, Jigsaw, Thermo, Windoku.
Try it on an Extreme Arrow puzzle. Next Step points it out when it is the move.