Jellyfish

A digit confined to four columns across four rows clears them.

Learn the Jellyfish Sudoku technique: the four-line fish — a digit boxed into four columns across four rows lets you eliminate it.

How to use it

1. Find four rows. Pick a digit and find four rows where it can only go inside the same four columns (each row uses two to four of them).
2. See the lock. Across those four rows the digit occupies four columns, one per row and per column, so those four columns are spoken for.
3. Eliminate down the columns. Remove the digit from every other cell in those four columns. The same pattern works with rows and columns swapped.