What are they for? Are there any recommendations?
closed as not a real question by Dennis, DragonLord, Kyle, Canadian Luke, 8088 Jan 16 '13 at 6:13
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Basically it raises (+%) or lowers (-%) amount of power that your card is allowed to consume before it starts to throttle.
I guess anything higher than default (0%) is not safe, but so is overclocking too. If your cooling system on card is good, I think you can get a slightly better results while OC-ing.
The good example of how Power Control works can be found here. But I think it can vary greatly depending on a card, esp. between generations.