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 a Windoku board this works exactly as in Classic. The examples are from Windoku puzzles on this site.
Worked examples
A position from a real Windoku puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
Two rows, two columns
Look at the columns. Where does digit 7 appear?
Where it fits
ExtremeX-Wing is an Extreme technique: Extreme Windoku puzzles are the first to need it.
The Classic X-Wing lesson teaches it on a plain 9×9 board. The Windoku overview explains the rule and the levels.
X-Wing in the other styles: Sudoku X, Jigsaw, Thermo, Arrow.
Try it on an Extreme Windoku puzzle. Next Step points it out when it is the move.