Well, the AMD's Power Now IS supported, as well as Intel's SpeedStep.
I know that some functions are supported, with some Intel processors. I have been completely unable to get much of anything (Power) to work with AMD, other than simple power off (Asus M3A78-EM motherboard, with quad core Phenom processor). I would love to be able to throttle down my desktop. It has two available states, according to one of the utilities, but nothing changes anything. From what I understand, there is a program that might work, but I have been unable to locate such a thing.
I have been able to use setgetState.exe to change the clock on my Thinkpad T43 (1871-W8M - Intel, with 4 states available), which makes a tremendous difference in temperature (according to the Reactor temperature widget), and in how long the battery will last (according to the Beer Battery widget). Performance is quite acceptable, even at the slowest speed. Throttling, on the other hand, is pretty flaky. It claims that it can be throttled to 13% in steps of 13%, but very little actually works, if I throttle it below 50%. Doing that doesn't make a lot of difference anyway, and it seems to work well at 50%. None of that works with ACPI 3.14, as comes with eCS 2.0 RC6a. ACPI 3.16 works , but seems to have a few other problems. ACPI 3.17 is very broken on all of the systems that i have tried it on. ACPI 3.18 works about the same as 3.16, on my desktop (which isn't good), but I haven't found the time to try it on the laptop, yet.
I have also been unable to get my T43 to resume. It appears to suspend properly, but I can't get it to wake up again. The screen comes on, but that is all. There is no display, just the background light. The only thing that I can do, is hold the power button, until it goes off. I haven't spent the time to try every possible combination with it, so I might have some settings that need to change.
I can see that ACPI is going to be necessary, someday, but it is barely usable, even on Intel machines (AMD is supposedly not supported, which is not acceptable). The main problem seems to be that the ACPI specs are not properly documented, and every manufacturer takes a different approach, with every machine they make, trying to get it right. Obviously, Microsoft totally ignores that stuff, and does it themselves. The ACPI team is trying to do it according to the specs, and that just doesn't work, because nobody seems to know how what the specs actually mean.
The original question was:
I understand it can help reducing power consumption, but how?
The answer seems to be that SetGetState.EXE, and SetGetThrottle.EXE, are the available tools. IF ACPI actually supports them, on your specific machine, they should help. If not, you are left with the option of reporting the problems to the ACPI developer, and hope that they will figure it out, someday.
FWIW, I get the impression that Mensys realizes that ACPI is a major problem. Whether they can do anything about it, or not, depends on a lot of things. One of those things is finding people who are willing to do a little work to report successes, and failures, to the ACPI developer. Unfortunately, that also means the users must have a valid Software Subscription (so you can get the software to test), and they need to do some serious reading, of poorly written, and sometimes misleading, and confusing, documentation. It also takes a LOT of time to try all of the possible combinations, to see what does, or doesn't, work. If you are not sure what you can do to help the project along, start by reading the docs (more than once), and start trying things, to see what works, and doesn't work. Then report your findings (after seeing if somebody else has reported the problems, so you don't duplicate things).
Have fun...