When two digits in a region can go only in the same two cells, those cells hold nothing else.
Where to lookA pointer first: in row 1, 2 digits can only go in the same 2 cells.Hidden pairIn row 1, 2 and 5 can only go in column 7 and column 9, which fills those cells. So they can't be 1 or 9.
Pick two digits in a row, column or box and see which cells each can still go in. If both are limited to the same two cells, remove every other candidate from those cells.
This example, step by step
Where to lookA pointer first: in row 1, 2 digits can only go in the same 2 cells.
Hidden pairIn row 1, 2 and 5 can only go in column 7 and column 9, which fills those cells. So they can't be 1 or 9.
In hints
The hint first gives a direction, a region. Blue ground marks the region, blue marks the two digits kept in the two cells, and orange strikes remove the other candidates.