Y-Wing
The Y-Wing, also called the XY-Wing, is built from three cells that each hold exactly two candidates. One cell is the pivot, holding digits A and B. It sees two pincers: one holding A and C, the other holding B and C. Whichever digit the pivot takes, it knocks A or B out of a pincer, forcing that pincer to be C. So one of the two pincers is always C — and C can be removed from every cell that sees both pincers.
Concretely: let the pivot r5c5 hold 2/7, with pincers r5c9 holding 2/4 and r2c5 holding 7/4. If r5c5 is 2, then r5c9 loses its 2 and becomes 4. If r5c5 is 7 instead, r2c5 loses its 7 and becomes 4. There is no third option, so a 4 stands in r5c9 or r2c5 — we just don't know which. The cell r2c9 sees both pincers (sharing a row with one and a column with the other), so r2c9 cannot be 4.
Note what the pivot does and doesn't tell you: the pivot itself is never resolved by the technique, and the eliminated digit C is the one the pivot does not contain. The three cells must be genuinely bivalue, and "sees" means sharing a row, column or box — pincers connected through a box are just as valid as those on lines.
Look for Y-Wings once your grid is well marked and bivalue cells are plentiful. Start from any bivalue cell as a candidate pivot and check the bivalue cells it sees for the AC/BC pattern.
- Catalogue your bivalue cells first — pivot and both pincers must have exactly two candidates.
- The pattern is A/B pivot, A/C and B/C pincers: the shared pincer digit C is the one you eliminate.
- Eliminations land only on cells that see both pincers — box connections often yield the extra victims solvers miss.