Hidden Pair
The hidden pair is the twin of the naked pair, thought the other way around: it is not the cells that reveal the pair, but two digits that are possible in a unit only in the same two spots. See below how it clears out the cells.
Interactive example
Click ‘Step’ to see the pattern and the elimination.
Try it: Hidden Pair
How the hidden pair works
Find two digits that appear as a candidate in a row, column or block only in the same two cells, for example 2 and 8. Those two cells often hold more candidates, but because 2 and 8 must go here, all other candidates in these two cells are impossible. Remove them and what remains in both cells is exactly the hidden pair. After that it acts like a naked pair.
More techniques
Naked Pair · X-Wing · Y-Wing