Yep, Eliovp is right.
Its not the real "THE Unvervolting" but its just thing when gpu can use powerstage from for example 900MHz and overclock this stage to 1100MHz.
It works as undervolt, there are positive results but i also would not call this undervolt.
So i made corection, now its called "Powerstage"

Anyway it can do little more magic but still the full power is in custom bios.
Here is list what can be done with GPU bios using hex editor:

This can do anyone:
1a) overclock
1b) lowering powerstage in my OS
2a) simple copying timings form 1500 to higher values via PolarisBiosEditor

This can do only pro like Eliovp, wolf0 and few others:
3a) adding voltage offset (if its not present already)
3b) changing offset to lower and lower values (this is more less simple opreation)
2b) applying true custom timings (if you have the timings its simple, hard is to make those )