If a particular candidate is present in only two Cells in two Rows and if these Cells belong to the same Columns (forming the corners of a rectangle), then whichever Cell the candidate is the solution for in the first Row, the candidate must be the solution for the Cell of the second Row that is located in the other Column (diagonally in the rectangle).

As this candidate must be the solution in two opposed corners of the rectangle, it can not be the solution anywhere else in these Columns.

The reasoning is also applicable when you replace "Row" by "Column" and "Column" by "Row".

In the example above if candidate 9 is the solution in B4, then it can not be the solution in any other cells in Column "4" and it must be the solution in H7 which implies that it can not be the solution in any other cells in Column "7". Conversely, if it is the solution in B7, then it can not be the solution in any other cells in Column "7" and it must be the solution in H4 which implies that it can not be the solution in any other cells in Column "4".

You can practice this strategy by installing the SudokuCoach application on your Android™ device.