Find a digit that appears as a candidate in exactly two cells of one row. Then find another row where the same digit also has exactly two candidates — and where those candidates sit in the same two columns. The four cells form the corners of a rectangle.
Each row must contain the digit exactly once, so one corner in each row is the real one, and they must be on opposite corners. Whichever diagonal it turns out to be, both columns end up containing the digit inside the rectangle.
That means the digit cannot appear anywhere else in those two columns. Erase it from every other cell in both — which is often four or six eliminations from one observation.
The pattern works the other way round too: two columns with two candidates each, sharing two rows, clears those rows.