A W-wing starts from two cells showing the same two candidates — say {3,6} — that do not see each other. On their own they prove nothing. What joins them is a strong link: a house where one of their two digits has exactly two possible cells, each aligned with a different twin.
The linking house must place that digit at one of its two ends. Whichever end takes it, the twin that end sees loses the digit and collapses to the other one. So one of the two twins certainly holds the other digit — and any cell that sees both twins can never hold it.
To spot one, note the grid’s bivalue cells as you go; matching pairs are common late in a puzzle. The work is finding the bridge: a row, column, or box where one of the pair’s digits is down to two cells, one seeing each twin. When it exists, the twins act like a naked pair stretched across the grid.
On the board
The twins here are R1C1 and R5C7, both {3,6}. The bridge is row 2, where the digit 6 fits only at columns 3 and 7: R2C3 shares the top-left box with R1C1, and R2C7 shares column 7 with R5C7. If row 2 puts its 6 at C3, then R1C1 cannot be 6 and must be 3; if at C7, then R5C7 must be 3 instead. One twin is a 3 either way — and R5C1, sharing column 1 with one twin and row 5 with the other, loses its 3, dropping from {3,5} to a bare 5.
This is the exact board the app's interactive W-Wing demo uses — step 32 of its 76-step walkthrough, drawn here with its live pencil marks.
