Quads
How it works
Four digits, within one row, column or box, only fit into the same four cells — even though those cells may still show other candidates too. Since all four digits have to go somewhere in that unit, and only these four cells can take them, every other candidate can be cleared from those four cells.
Try it
No worked example for this one yet. It turns up rarely enough that no puzzle has needed it as its next step so far — one will appear here as soon as a generated puzzle offers it.