What does the statement, "effect of stress cannot reduce FS from value computed without stress", mean?

On page 23 of the Unwedge Theory Manual, in the section on Field Stress theory, it states the following:

It should be noted that the effect of stress cannot reduce the factor of safety from the value computed without stress in section 7.  The reasoning for this is that once any movement of the wedge occurs, contact with the rock mass is lost, and the factor of safety reverts to the unstressed value.  As a result if stress is included in the analysis, both the unstressed and the stressed factors of safety are calculated and the maximum of the two is reported.

If the effect of stress is causing a decrease in factor of safety, then the reported factor of safety will stay the same (i.e. the program will be reporting the unstressed factor of safety).  If you only want to report the stressed factor of safety, go to the Field Stress option and select the Advanced option.  Toggle on the option to report only the stressed factor of safety.  In this way you can see what the stress is doing to the factor of safety.

 Unwedge-factorofsafety.jpg

Change Advanced Settings for Stressed to see the effect of stress on factor of safety