Look at one digit inside one box. If all of its candidate cells happen to sit in the same row — two or three of them, it does not matter which — then the digit is somewhere in that row, inside this box.
That is enough to act on. Wherever it lands, it is in this box and in this row, so it cannot be anywhere else along the row. Erase it from the row's cells in the other two boxes.
This is the first technique that reasons across two units at once, and it is the bridge between scanning and real pattern work. It also runs in reverse — a digit confined to one line within a box, seen from the line's perspective, is the box-line reduction.