An X-Wing across three rows. When a number's places in three rows all fall in the same three columns, those rows take all three columns' copies, and the number leaves the rest of those columns.
123456789
123456789
5
8
3
9
2
147
146
4
2
9
5
2
9
4
6
5
8
3
9
2
5
4
3
6
8
9
8
6
2
5
8
5
6
1
147
2
3
147
9
6
9
5
3
2
3
8
1
2
5
4
6
9
7
2
9
5
6
3
7
8
14
14
The three rows' placesLoses a pencil mark
Not every corner is needed
A Swordfish needs only that each row's places fall in the three columns. Here each row has two places, not three, and it still works.
How to spot one
Count a number's places in each row. Three rows with two or three places each, all inside the same three columns, make a Swordfish.
What it gives you
Removals in the three columns, outside the three rows. Turned on its side, three columns clear three rows.
Practise Swordfish
Boards that need it, with the owl's hints when you're stuck.