The XY-Wing links three cells that each hold exactly two candidates. A pivot with digits XY sees two pincers: one with XZ, one with YZ. Whichever digit the pivot finally takes, X or Y, it pushes one of the pincers to Z. Since some Z is certain, any cell that both pincers can see is barred from holding Z — that digit would contradict whichever pincer becomes Z.
Pivot 2-7 at r4c4, pincers 2-9 and 7-9 — the 9 in r8c8 is seen by both pincers and dies.
Pivot r4c4 holds candidates 2 and 7. It sees pincer r4c8 with 2-9 and pincer r8c4 with 7-9. If the pivot resolves to 2, r4c8 is forced to 9; if it resolves to 7, r8c4 is forced to 9. A 9 is coming either way — so r8c8, which both pincers see, cannot hold 9, and any 9 pencilled there is removed.
All three cells must be strictly bi-value, and the pincers must NOT see each other for the classic pattern. A pivot with three candidates makes it an XYZ-Wing with a much smaller elimination zone.
These grids come from rated games on this site — our solver confirmed that xy-wing was part of the shortest solving path for each one.
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