Pair elimination
A letter/digit pair already used elsewhere cannot be reused
How to use it
The mirror of the pair hidden single, asked at a cell. For each letter candidate of a cell, check the pairs it could form there: pair it with every digit candidate. If every such pair is already used on the grid or already ruled out at this cell, the letter cannot stay — strike it. Symmetrically for digits.
This is mechanical propagation rather than a named insight: the board applies it automatically after every move and after every block, which is why the walkthrough interleaves it as small "ruled out by orthogonality" beats between placements mid-solve — and, when the givens' own fallout starts the solve, as one opening beat ahead of everything else. Watch for it after placing a pair with popular symbols.
Why it is sound
By orthogonality every (letter, digit) pair occurs exactly once in the grid. So once a pair is placed, it is gone from every other cell; and if every pair available to a letter (or digit) at a cell is already used or blocked, that letter (digit) cannot remain there. Removing it only discards configurations absent from every solution.
A worked example
Before any named technique, the givens themselves kill candidates by orthogonality. The pair A4 sits at r1c1, and no other cell may ever carry it. Two cells here already hold the letter A as a given — r3c4 and r5c3 — so their digit 4 could only ever complete the used pair A4. Used once, used up: the 4 is struck at both cells in one sweep. Each such removal is a pair-elimination: a value whose every pair is unavailable cannot stay. The walkthrough shows this opening sweep as its own beat, then the real solving begins on the pruned board.