Hidden Triple

The pattern

For each digit still missing from a unit, write down the cells where it could go. If three digits have footprints that all fit inside the same three cells, those cells are reserved for them.

Each of the three digits does not need to be possible in all three cells — a footprint of two cells is fine, as long as all three footprints stay inside the same set of three.

The cleanup

Erase every candidate other than those three digits from those three cells. Often that turns a cell with six candidates into a cell with two, and the unit that looked hopeless becomes a naked triple you can then use against the rest of the unit.

Why it is hard to see

Because nothing shrinks visually until you act. A naked triple shows itself in short candidate lists; a hidden triple can sit inside three of the busiest cells on the board. The only reliable way to find one is to count positions per digit rather than read candidates per cell.

This is why hidden subsets show up on Expert here: the level requires three or more elimination steps, and hidden ones are where the extra steps usually come from.

Why it is valid

The unit needs all three digits, and all three can only live in these three cells. Three digits into three cells uses them up completely, so any fourth candidate in those cells has nowhere to be placed.