Locked candidates
How it works
Box/line reduction — the mirror of pointing. Look at one row or column: if every remaining cell that could hold a digit falls inside a single 3×3 box, that digit must end up somewhere in that box — so it can be eliminated from the rest of the box outside that row or column.
Try it
Look at column 1. Let's pencil in what's still possible there, then see why digit 3 is forced into just one box.
5
1
2
8
6
7
3
9
4
8
9
5
4
1
2
6
6
4
9
2
5
8
7
4
7
9
6
2
1
9
6
4
2
7
7
2
6
4
1
6
2
4
6
4
2
7
8
2
5
1
4
8
6
7