If one can identify a group of four Cells containing various combinations of only the same four Candidates and if each of these four Candidates can see all the other Cells of the group where it is present, except for one of those Candidates, then this Candidate can not be the solution in any Cell outside of the group that can see all the Cells of the group where it is present.

Indeed, if this Candidate were the solution in a Cell outside of the group and if that Cell saw all the Cells of the group where this Candidate is present, then it would eliminate all occurences of this Candidate from the group. The group of four Cells would then contain only three possible Candidates that can each be the solution in only one Cell of the group, leaving one Cell of the group without solution.

In the example if candidate 9 were the solution in F7, then candidate 4 would be the solution in F6, 5 in E7, 8 in E9... and E5 would be empty.

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