The first thing you need to understand about PBO is that it is not overclocking.
PBO, as the name sort of hints, is an extension to Precision Boost 2. The only changes that PBO makes are to power restrictions. There are three power metrics that prevent a Ryzen or Threadripper processor from clocking higher. One is PTT, which is Package Power Tracking. That's the number of watts that the CPU is actually consuming, per the information provided by the motherboard. The second is TDC, or Thermal Design Current. This is the sustained current drawn from the motherboard VRM's. The third is EDC, or Electrical Design Current, which is the maximum current that can be drawn in a transitory manner from the motherboard's VRM's - put another way, the largest spike in current that will be allowed.
Setting these values higher allows the processor to consume more power, but leaves the clock speeds and voltage entirely up to the processor to determine.
As a general rule, if you want to use PBO, you should have a good cooler, and should set the three values to the max supported by the BIOS you are using. This effectively means that the CPU will be limited not by power usage, but by clock speeds (it will not boost above its rated speed) and temperature (it will not continue to boost if it's reached max temperature).
If you want to fiddle around with settings, do a manual overclock. If you want to leave the fiddling to the engineers who created the CPU, then enable PBO and set PPT, TDC, and EDC to the maximum allowed values. That will give you the best non-overclocked performance available from the CPU, without sacrificing single-core boost speeds (which you lose with a manual overclock).
