- Look at the highlighted cells.
- Look at their notes: whichever way they are filled, the same notes are ruled out.
- So 2 can be removed from the crossed-out cells.
XY-Wing in sudoku
An XY-Wing is made of three cells with two notes each. The pivot holds X and Y. It sees two other cells, the pincers: one holds X and Z, the other Y and Z. The pincers need not see each other. Every cell that sees both pincers loses Z.
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How to spot it
Look for a cell with two notes, and among the cells it sees for two more with two notes, each sharing a different number with the pivot and the same third number with each other. Then find the cells that see both pincers. Puzzles with many two-note cells are where to look.
Why it works
The pivot is X or Y. If it is X, the pincer with X and Z must be Z. If it is Y, the other pincer must be Z. One of the pincers is Z in either case, so a cell that sees both of them cannot be.
On a real puzzle
This is XY-Wing on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Extreme puzzle calls for XY-Wing. Fill in the singles first. When no number is obvious any more, look for the pattern, or ask for the hint: each game has one.