Pair pointing
A pair confined to one row or column cannot pair with other symbols there
How to use it
Take one unused pair, say C3, and look at all its remaining homes. If they all lie on a single row (or column), the pair is confined to that line — and then, inside that line, C pairs only with 3 and 3 only with C. The lock has two effects: at the other cells of the line, C cannot pair with any other digit and no other letter can pair with 3; and C itself is stripped from the line's cells outside the homes, since the line's C lives at one of them.
Check pairs whose homes cluster during a stall: a letter nearly pinned along a line often leaves one pair with two or three homes, all on it. The blocked combinations frequently kill a candidate elsewhere by pair elimination.
Why it is sound
If every remaining home of a pair (L, d) lies on one line, then in every solution the pair sits on that line, so the line's L and its d are both consumed there. Inside the line no other cell may pair L with another digit, nor another letter with d (each would duplicate the line's L or d); outside the homes the letter L cannot appear in the line at all. Both consequences are pure Latin-plus-orthogonality facts, so the blocks and eliminations are sound.
A worked example
Where can the pair B3 still go? Its letter B survives in column 3 (rows 2 and 4, via the placements at r1c1 and r3c2) and its digit 3 survives in the same column — so every remaining home of B3 lies in column 3. The pair is confined there, which locks the column: inside column 3, B pairs only with 3 and 3 only with B. At r2c3 and r4c3 the other combinations (B1, B2, B4, C3, ...) are blocked, and the fallout is immediate: with B1 ruled out at r2c3 and C1 already placed at r1c2, the digit 1 has no admissible pair left there and dies. One confinement, several collapses.