GideonTech.com posted a review on the Noctua NF-S12 & NF-R8 Fans

The U.L.N.A (Ultra-Low-Noise Adaptor) is a power cable that has a resister in line with the fan, cutting the amount of power that the fans uses, which slows it down. If you use those, there is not a way to speed up the fan without powering it down and removing it.

Noctua NF-S12 & NF-R8 Fans Review