How to solve a WXYZ-Wing in Sudoku
A WXYZ-Wing is four cells holding four digits between them, where one digit must land in at least one of them, clearing it from cells that see them all.
How it works
Four cells together contain only the digits W, X, Y and Z. For three of the digits, every cell holding that digit sees every other cell holding it. Only one digit, Z, is not so restricted.
If none of the cells were Z, the four cells would have to hold just three digits, so one digit would appear twice in cells that see each other, which is impossible. So one of the Z cells is Z, and any cell that sees all of them cannot be.
How to spot it
Start from a cell with up to four candidates and find three cells it sees that use no digits other than those four.
A worked example
This is a step from a real puzzle, a moment where the next move needs a WXYZ-Wing.
P is the pivot, W are its three wings. Struck-through digits are the ones that can be removed.
R4C2, R4C7, R5C2, R7C2 hold only 2, 4, 5, 7 between them, and every digit except 5 can only be in cells that see each other. So they can't all avoid 5: one of R4C7, R7C2 is 5. Remove 5 from cells that see all of them: R7C7.
Where it fits in Rocksled Sudoku
Puzzles that need a WXYZ-Wing are never rated below Expert. Each step of this kind adds 72 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.