Pairs
How it works
Two digits, within one row, column or box, only fit into the same two cells — even though those cells may still show other candidates too. Since both digits have to go somewhere in that unit, and only these two cells can take them, every other candidate can be cleared from those two cells.
Try it
Look at row 1. Let's pencil in what's still possible for Row 1, Column 5 and Row 1, Column 6, then see why 2 and 6 are hiding there.
5
1
3
6
2
1
1
4
2
7
3
9
5
6
8
9
9
7
4
1
1
7
3
8
9
7
6
6
5
8
4
1
4
3
8