Strategy Detailed Descriptions Strategy Detailed Descriptions This is sometimes referred to as "Hidden Candidates".
Hidden N-tuplets are when exactly N numbers are only Possible in exactly N cells.
Note that the N-tuplet numbers must be impossible in all other cells of a box, row, or column; however non-N-tuplet numbers can be Possible in the N-tuplet cells.
When an N-tuplet is found, then all non-N-tuplet numbers in the N-tuplet cells are marked Impossible. Furthermore, if the N-tuplet cells are all in one row or column and all in one box, then the N-tuplet numbers can be marked Impossible in other cells of the intersecting group (they would be found by the regular N-tuplet strategy).
In the row above, numbers have been marked impossible (details from other rows and boxes not shown). The numbers 5 and 6 are only possible in the first and fourth cells, so they form a hidden couplet (2 numbers that are possible in only 2 cells). This means that all other numbers besides 5 and 6 can be marked impossible in those two cells.