A Hidden Pair is the shy twin of a Naked Pair.
You are not looking for cells that *show* only two candidates. You are looking for two digits that, inside one house, can live in only the same two cells — even if those cells are cluttered with extra pencil marks.
Why it works
Those two digits must both appear in the house. They only have two shared seats. So those two seats must be exactly those two digits (in some order).
Any other candidate written in those cells is a decoy: if one of those cells became a different digit, one of the pair-digits would have no seat left in the house.
After you clear the decoys, the Hidden Pair becomes a Naked Pair — same ownership idea, found from the digit side instead of the cell side.
How to spot it
Track digits, not cells. Box-candidate notation makes Hidden Pairs easier to spot. After you strip extras, you often have a Naked Pair ready to clean the house.
Demonstration
In one house, track where each digit can still go. You are hunting digits, not staring at one cell.
Here 2 and 4 both appear only inside the same two cells. Shrink their homes further… until those two seats are the only ones left. That shared lock is a Hidden Pair.
Those two cells must hold 2 and 4 and nothing else — otherwise one of the digits would be homeless in the house.
Remove the extra candidates from those two cells (here 7) (red). You are uncovering the pair so the house’s digit-debt can be paid — and often revealing a Naked Pair next.
Example 1
Find two digits that appear only in the same two cells of a house.
7 and 9 form a Hidden Pair — they share exactly two seats.
Clear other candidates from those cells (here 5 and 6) (red). The seats must be reserved for the pair — otherwise a digit would have nowhere to go.
Example 2
Find two digits that appear only in the same two cells of a house.
3 and 8 form a Hidden Pair — they share exactly two seats.
Clear other candidates from those cells (here 2, 5, and 9) (red). The seats must be reserved for the pair — otherwise a digit would have nowhere to go.