When using offset you are adding or subtracting voltage from the cpu and are only using small values.
Let me give you an example of how offset works.
Say I have my 3770k at 4.5GHz and 1.35v is needed to be stable. I open up cpu-z and it's showing 1.35v all good, so I run a stress test and during the stress test cpu -z is showing it jumping up to 1.38v.
This is where offset comes in, you just enter (negative with the minus sign) -.030v in the offset and run the test again to see how close it's staying to 1.35v and that's pretty much it.
keep in mind as you read in those guides the offset affects idle voltage too so if you take it too far you will get a bsod because it doesn't have enough voltage for stability at idle.
I hope I didn't make this sound too confusing but that's pretty much all there is to using offset.