The X-Wing is a rectangle of 4 cells which all contain a given candidate. In a given set of 2 rows, the candidate must appear only twice and in the exact same 2 columns. Similarly, in a given set of 2 columns, the candidate must appear only twice and in the exact same 2 rows.
Because the candidate appears exactly 2 times in each row and there are 2 rows, it means that the candidate must appear exactly twice in these 4 cells. Therefore it cannot apear anywhere else in the 2 columns. Similarly for the column based X-Wing.
7 forms an X-Wing on rows 2 and 6.
That means that candidates 7 can be removed from all other cells in columns 6 and 7, cell [1,7].
7 forms an X-Wing on columns 2 and 8.
That means that candidates 7 can be removed from all other cells in rows 3 and 4, cells [3,5] and [4,1].
If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.Sample Puzzle 1 Sample Puzzle 2 Sample Puzzle 3