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 digit takes one corner of each column. The rest of those 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.
Two rows, two columns
Look at the rows. Where does digit 3 appear?
Where it fits
ExtremeX-Wing is an Extreme technique: Extreme Sudoku X puzzles are the first to need it.
The Classic X-Wing lesson teaches it on a plain 9×9 board. The Sudoku X overview explains the rule and the levels.
Try it on an Extreme Sudoku X puzzle. Next Step points it out when it is the move.