If you set the mV Boost box to 100, it will boost the CPU voltage by +100 mV at 800 MHz.
The problem when undervolting is an offset undervolt of -100 mV reduces the CPU voltage at every frequency. Reducing the max speed voltage is good but reducing the low MHz voltage can cause stability problems. The mV Boost feature can help with that problem. It lets you undervolt the full MHz voltage without having to worry that the low MHz voltage will cause instability.
This forum thread shows a voltage curve example for this feature.
What value of this variable is best for the i9-13980HX CPU. The CPU Core value used is -150 mV.
www.techpowerup.com
Some more info.
This is likely by design and I am just not viewing it with the correct context, but when I have -50.8 mV undervolt for CPU Core and CPU P Cache, and 100 set for mV Boost, it displays as 0.100 V in HWiNFO64. I guess that fact that it is showing an offset is good for knowing that my undervolt was...
www.techpowerup.com