If a digit can go in only two cells in each of two rows, and those cells line up in the same two 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.
Two rows, two columns
Look at the columns. Where does digit 8 appear?
Where it fits
ExtremeX-Wing is an Extreme technique: Extreme Arrow puzzles are the first to need it.
The Classic X-Wing lesson teaches it on a plain 9×9 board. The Arrow overview explains the rule and the levels.
X-Wing 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.