XY-wing
A bivalue pivot and two bivalue pincers eliminate their shared candidate
How to use it
A three-cell sling. Find a pivot with exactly two candidates, x and y, and two pincer cells that see the pivot (share its row or column), one holding {x, z}, the other {y, z}. If the pivot takes x, the first pincer takes z; if it takes y, the second does. Either way one pincer is z — so any cell that sees both pincers cannot be z.
The pivot and pincers are bivalue cells: harvest them during a stall (cells with two candidates on one layer) and look for a shared z. Eliminations land on the common peers, often several at once.
Why it is sound
The pivot holds {x, y}; the pincers, seeing the pivot, hold {x, z} and {y, z}. In every solution the pivot is x or y. If it is x, the first pincer is z; if it is y, the second is z. So every solution has z in one of the two pincers, and a cell seeing both can never be z.
A worked example
The pivot sits at r3c4 with digit candidates {4, 5}; two pincers see it — r4c4 in the same column, r3c3 in the same row — and each pincer holds digit 1 as one of its two candidates. If the pivot is 4, the pincer sharing that 4 must release it and show its 1; if the pivot is 5, the other pincer does. Either way one pincer is a 1, so every cell seeing both pincers loses digit 1: it dies at r4c3.