If a particular candidate is present in only two Cells in a Column and if it is only present in the same Rows of another Column (forming the corners of a rectangle) plus Cell(s) of this second Column that belong to the same Square as one of the corners of the rectangle (these extra-cells are called the "fin"), then all candidates present in the same Square as the "fin" and in the same Row as the rectangle corner linked to the "fin" can be eliminated.

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

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

In the example above if candidate 4 is the solution in B8 then candidate 4 must be the solution either in D3, or in the "fin" (in this case Cell D1), which eliminates it from all other Cells in Square "4" (and in particular from E1). If it is the solution in B1, then it eliminates all other candidates 4 in Column "1" (in particular from E1). Whichever the solution for candidate 3 in Row "B", it can not be the solution in E1.

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