When two digits can go only in the same two cells of a house, those cells must hold them, and their other candidates can go.
On an Arrow board this works exactly as in Classic. The examples are from Arrow puzzles on this site.
Worked examples
A position from a real Arrow puzzle on this site. Step through it: the board lights what each step talks about, and the red lines show what rules what out.
Two digits, two cells, in a box
In box 1, look for two digits that fit in only two cells.
Where it fits
HardHidden Pair is a Hard technique: Hard Arrow puzzles are the first to need it.
The Classic Hidden Pair lesson teaches it on a plain 9×9 board. The Arrow overview explains the rule and the levels.
Hidden Pair in the other styles: Sudoku X, Jigsaw, Thermo, Windoku, Kropki.
Try it on a Hard Arrow puzzle. Next Step points it out when it is the move.