If a particular candidate is present in only three Cells in three Rows and if these Cells belong to the same Columns, then whichever Cells the candidate is the solution for in the first two Rows, these Cells must be in different columns and the solution for the third Row must be in the third Column. Hence, the candidate 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".

The candidate must not be present in all Cells of the 3x3 pattern, as long as all allowed combinations of the Candidate present in the pattern result in the Candidate being the solution in each Row and in each Column of the 3x3 pattern.

In the example above the swordfish is made of the Cells where candidate 1 is colored in green, plus B5, E1 and F7.
Whichever possible combination for candidate 1 is built in Rows "B", "E" and "F", candidate 1 must be the solution in one of the Cells in Columns "1", "5" and "7" of these Rows. Thus all candidates 1 in Columns "1", "5" and "7" can be deleted except in Rows "B", "E" and "F".

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