XYWing
An XYWing is a very advanced technique which can eliminate certain candidates from squares. Its name comes from the pattern formed by the numbers and is not related to XWing.
We are looking for three squares, each of which contains exactly two numbers. The first square contains XY and we require a buddy of this square to contain XZ. We then require another buddy of the XY square to contain YZ.
Any squares that are buddies to both XZ & YZ cannot contain Z. Why? Because it doesn't matter whether the XY is X or Y, the Z will be in either XZ or YZ and hence can be removed from their buddies.
In this Sudoku X=5, Y=7 & Z=3. R3C9 is XY. R6C9 is XZ. R3C5 is YZ. So no buddies of both R6C9 and R3C5 can contain <3>
In this Sudoku X=2, Y=5 & Z=3. R4C6 is XY. R6C5 is XZ. R7C6 is YZ. So no buddies of both R6C5 and R7C6 can contain <3>.
