Have you googled for it?
Results 1 - 10 of about 2,070 for amd "athlon 64" linux kernel power speed governor.
It is not difficult, but it requires a bit of studying most likely a kernel compilation. Once you got it, it is easy to understand.
Also it helps to download the datasheet for your AMD processor from www.amd.com.
The athlon contains certain registers which reduce the clock. The appropriate kernel driver knows how to communicate with this registers. The governor measures the activity and ask the driver to reduce/increase speed.