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

Jellyfish

The same number is boxed into four lines, so it can leave those columns elsewhere.

An example from a real puzzle

How to spot it

1. Find a number whose places in four rows fall within the same four columns.
2. Remove the number from every other cell of those four columns.

Why it works

Four rows need four copies inside four columns, one per column, so those columns have no room for the number anywhere else.