Sudoku Technique
XY-Wing is useful when the grid has many bivalue cells. Instead of looking for a rectangle, you look for a pivot cell and two wing cells that force the same candidate outcome.
An XY-Wing uses three cells. The pivot has candidates X and Y. One wing has X and Z. The other wing has Y and Z. If the pivot becomes X, one wing must become Z. If the pivot becomes Y, the other wing must become Z. Either way, one wing will contain Z.
Any cell that can see both wings cannot contain Z, because one of those wings must use Z.
XY-Wing is powerful because it does not need a neat line pattern. It uses relationships between cells, which makes it especially helpful on sparse puzzles and late-stage evil sudoku boards where line-based patterns are hard to find.