Fish
How it works
Pick a digit and two rows where it has only two possible cells left. If those two pairs sit in the very same two columns, the digit takes one cell in each row — and between them they use up both columns. So the digit can be eliminated from those two columns everywhere outside the pattern. It works identically starting from two columns and eliminating along rows.
Try it
Digit 7 is pinned down by rows 6 and 9. Let's pencil in the highlighted cells — every place 7 can still go in those lines — then see why it can't sit anywhere else in columns 1 and 4.
1
2
7
3
3
2
1
7
1
3
9
6
2
6
3
9
8
5
9
2
6
3
3
6
2
4
9
1
5
3
7
3
5
2
3