A hidden pair is two digits that, within one house, can only go in the same two cells. Those cells must hold that pair — one digit each — so every other pencil mark in them can be erased. The pair is “hidden” because the two cells look busy with other candidates until you sweep the house digit by digit.
To spot one, run each unsolved digit along a house and note its possible cells. When two digits share the same two cells and appear nowhere else in the house, you have the pair — clean out the rest of those two cells.
It works the same way a hidden single does, doubled: the house must place both digits, and it only has two cells that can take either. If one of those cells held some third digit, the house would be left with one cell for two digits.
On the board
Sweep row 9. The digit 5 can only go at R9C3 or R9C8, and the digit 7 can only go at those same two cells — the other empties in the row (columns 4, 5, and 6) allow neither. So R9C3 and R9C8 must hold the 5 and the 7 between them, and their other pencil marks are noise: the 3 is struck from R9C3, and the 1 and 9 are struck from R9C8. Both cells come out of it as a clean {5,7} pair.
This is the exact board the app's interactive Hidden Pair demo uses — step 17 of its 60-step walkthrough, drawn here with its live pencil marks.
