The XY-Wing is composed of 3 interconnected cells. The achor cell is connected to both of the other cells by sharing a different group with each of them.
Because the cells are connected both by sharing a group with the achor cell and 1 candidate with each other, it means that the candidate in common between the 2 wing cells MUST be in one of these cells.
That implies that the cells which are in the intersection of the peers of the 2 wing cells, cannot have their shared candidate as a possible value.
7 forms an XY-Wing in rows 1 and 5.
That means that candidates 7 can be removed from cell [5,8].
8 forms an XY-Wing in rows 1 and 2.
That means that candidates 8 can be removed from cell [3,4].
6 forms an XY-Wing on columns 1 and 3.
That means that candidates 6 can be removed from cell [2,1].
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