Offset (in your case a negative offset) is added to the VID table of voltage the processor has. So negative offset is subtracted from the VID voltage. Your CPU has a series of voltages preprogrammed..the VID table..for different loads/frequencies.
So, if you set an offset this is added/subtracted from the table of voltages throughout the range from idle to load.
The idea is that you work out your stable manual voltage in BIOS for whatever OC you want. Run something like Prime or Realbench as a stress test and note load voltage in CPUz.
Then set an offset voltage to get this same CPUz load voltage using same stress test.
if you set a positive offset the maths is not straight forward since the offset is taken into account by the CPU but also thermal conditions current draw etc. negative offset is more direct in the mathematics but not a straight sum either so often the best approach is a bit of trial and error to get an offset value that gets you the CPUz load value you are looking for.
You have to be careful sometimes setting a negative offset that is quite large....since at idle the offset is applied and might lead to too low a voltage and instability at idle despite being OK at load.
