How to solve a W-Wing in Sudoku
A W-Wing is two cells with the same two candidates, joined by a strong link on one of the digits, which clears the other digit from cells that see both.
How it works
Two cells that do not see each other each hold exactly the digits A and B. A strong link on A connects them: a unit where A has only two places, with one place seeing the first cell and the other seeing the second.
One end of the link must be A. If it is the end that sees the first cell, that cell cannot be A, so it is B. If it is the other end, the second cell is B. Either way one of the two cells is B, so any cell that sees both of them cannot be B.
How to spot it
Find two matching two-candidate cells that do not see each other, then look for a unit that links them through one of the two digits.
A worked example
This is a step from a real puzzle, a moment where the next move needs a W-Wing.
W are the two matching cells; A are the two ends of the link between them. Struck-through digits are the ones that can be removed.
R6C7 and R7C9 both hold only 6/5 and can't see each other. In column 8, 6 can only go in R6C8 or R7C8, and R6C8 sees R6C7 while R7C8 sees R7C9. If R6C8 is 6, R6C7 must be 5; if R7C8 is 6, R7C9 must be 5. Either way one of them is 5, so remove 5 from cells that see both: R6C9, R8C7.
Where it fits in Rocksled Sudoku
Puzzles that need a W-Wing are never rated below Expert. Each step of this kind adds 84 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.