Jellyfish
The X-Wing idea with four rows and four columns.
A technique for Expert puzzles. Play a Expert puzzle
What it does
The X-Wing idea with four rows and four columns. Pick a digit: if in four rows it can only go in cells of the same four columns, those four rows place the digit in those columns, one each, so no other cell of the four columns can hold it.
How to spot it
Very rare. Choose one digit and find four rows where it has two, three or four places, all within the same four columns. The same works with columns and rows swapped.
What it lets you do
You remove the digit from every other cell of those four columns (or rows, if you started from columns).
Try it
Find the Jellyfish on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Expert level; any Expert puzzle may need it too.