Hidden Pair

A Hidden Pair occurs when two values appear as candidates in exactly two cells of a unit, and nowhere else in that unit. Those two cells must contain those two values, so all other candidates can be removed from those cells.

For each unit, count how many cells contain each candidate value. If two values each appear in exactly the same two cells, you have a Hidden Pair.

Since those two values can only go in those two cells, the cells are fully determined by them. Any other candidates in those cells are impossible and can be eliminated.