If a particular candidate is present in only two Cells in a Row and if you can find another Row where the two Cells in the corresponding Columns (the four Cells forming the corners of a rectangle) contain the candidate in the first one and a solution (either an initial value or an induced value) in the second one, and if the candidate is also present in Cell(s) of this second Row that belong to the same Square as the corner of the rectangle containing a solution (these extra-cells are called the "fin"), then all candidates present in the same Square as the "fin" and in the same Column of the rectangle corner linked to the "fin" can be eliminated.

Indeed, if the candidate is the solution for the Cell in that Column for the first Row, then it eliminates this candidate in all the Cells in the same Column; if it is the solution in the other Cell of the first Row, then it must be the solution in the "fin", which eliminates the candidate from all other Cells in the same Square.

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

In the example above if candidate 5 is the solution in F8, then it eliminates all other candidates 5 in Row "F" (in particular in F2 and F3) and candidate 5 must be the solution either in A1, or in the "fin" (in this case Cells E1). If it is the solution in A8, then it must be the solution in E1 which eliminates it from all other Cells in Square "4" (in particular in F2 and F3). Whichever the solution for candidate 5 in Column "8", it can not be the solution in F2 nor in F3.

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