Sudoku technique · First needed at: Fiendish ●●●●●●○○

Swordfish

The number is confined to three lines that touch only three crossing lines, which clears it everywhere else along them.

An example from a real puzzle

How to spot it

1. Find a number whose places in three rows fall within the same three columns — two or three places per row.
2. Remove the number from every other cell of those three columns.

Why it works

The three rows need three copies, and they can only land in those three columns, one each — so those columns have no room for it anywhere else.