Y-Wing

A Y-Wing (also called an XY-Wing) starts with a pivot cell that has exactly two candidates, say X and Y. Two more cells — each also down to exactly two candidates, and each sharing a row, column, or box with the pivot — act as pincers: one holds X and a third digit Z, the other holds Y and that same Z.

Whichever of X or Y the pivot turns out to be, one of the two pincers is forced to Z — if the pivot is X, the first pincer can't be X anymore and becomes Z; if the pivot is Y, the second pincer becomes Z instead. Either way, Z lands in one of the two pincers, so any cell that's a peer of both pincers can have Z eliminated as a candidate.

Suppose a pivot cell has candidates {2, 7}, one pincer sharing its row has {2, 9}, and another pincer sharing its column has {7, 9}. If the pivot is 2, the row pincer must be 9; if the pivot is 7, the column pincer must be 9 — so 9 goes in one of those two cells no matter what the pivot turns out to be, and any cell that sees both pincers can have 9 crossed off.