Step 22 · Master
What is W-Wing? How to Solve Killer Sudoku
Two cells that can only be X or Y and don't see each other, linked by a line where X has only two spots, let you remove Y from any cell that sees both of them.
What it is
Two cells can each only be 4 or 9, and they don't see each other.
There's also a row (column or box) where 4 fits in only two places, and each of those places sees one of the two cells.
If both cells were 4, that row would have nowhere left for 4. So one of the cells must be 9.
Any cell that sees both can't be 9.
How to spot it
Find two cells with the same two candidates that don't see each other, then look for a digit with only two spots that links them.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
R1C8 and R9C9 can each only be 2 or 6. In row 7, 2 fits only in R7C8 or R7C9; R7C8 sees R1C8 and R7C9 sees R9C9. If both cells were 2, row 7 would have nowhere left for 2. So one of them is 6. Any cell that sees both (R1C9 and R9C8) can't be 6.