After hunting this information all over the vast global internet I actually found it in a performance related word document from Microsoft. The enabling of CPU Core Parking is NOT done in a group policy like it says everywhere, so you can close that MMC right away.
Instead fire up CMD and enter the following:
powercfg -setacvalueindex scheme_current sub_processor bc5038f7-23e0-4960-96da-33abaf5935ec 25
This line will allow the CPU to park 75% of it’s cores. As in if you’ve got 4 cores, 3 can park. If you’ve got a dualcore the number to park one core would actually be 50 then. Clear on that? (Don’t ask me about the long sub_processor number thingie cause I have no idea) (UPDATE: Now I do know, check http://www.nullsession.com/2009/11/24/powercfg-unleashed/ for more info on that string)
powercfg -setactive scheme_current
This will activate the settings you just entered, in the currently active powerplan.
Did it work? Check out Resource Monitor for an answer. The more cores you have, the more likely anyone will be parked one could say.