I think it's like that:
For regular vCore undervolting you supply less V(oltage) and the F(requency) of the V/F curve drops with it. It also applies the same offset to all frequencies.
For PBO2 you adjust the V of the V/F curve. The F stays where it is. Temps then drop and the chip can boost higher momentarily. It sets different offsets to frequencies.


You can also set different limits for all cores after having tested which core performs how well.
Fairly easy to do on an 5600x, but you might spend weeks on an 5950x until it's stable, haha.

That's because the annoying thing is that the offset you set is applied for ALL loads, lesser for high loads and more for low loads.
And for idle or low loads it's just too much. Which is where you probably saw your reboots as well.
Example: -1 equals to ~ -3 to -5 mV. Probably -3 on high frequencies and -5mV on lower frequencies. If you set -10 in the PBO2 menu it'll be -30 to -50mV.



If you have a golden chip maybe you can make it to -30 (90/150) and be fine throughout curve...
but chances are at some place of the curve it's not enough Voltage supplied and hence you get a reboot.

-30 for all cores is utopian anyways, most chips won't even boot into windows with that, let alone be all time stable.
He surely has a nice binned chip supplied by AMD (why do most reviewers just don't get that?) but likely it's not even stable for him on all steps of the V/F curve.

Like SSJBen pointed out, this guy does too many mistakes in his 'guides' cuz he doesn't really get into stuff before recording his videos.