The main component of PowerDevil is a KDED module, and so it works as a daemon running in background. It will start upon KDE session startup, so you don't need to worry about anything. Configuration happens through a KCModule (eg. System Settings), where you can set up your profiles and some general configuration options.
PowerDevil is "profile based". This means you can create your own profiles and assign them to the correct situation. For manual changes, there is a nice KRunner. You can change the current profile, brightness, governor, and more through it. And a Plasmoid is coming, thanks to Sebastian Kügler and Riccardo Iaconelli, which will replace the traditional system tray you were used to.
Technically speaking, PowerDevil is 99% based on Solid. It uses a pluggable polling system, allowing the user to decide which method to use to catch idle events. Currently there is a method based on XSync, and one based on intelligent timers and a grabber widget. I will soon add an old school timer-based method, for maximum compatibility.
PowerDevil has just been imported into kdebase, and will be your fellow power manager from KDE 4.2 onwards. All suggestions, feedback, comments or whatever is really appreciated, you can get in touch with me through mail or IRC. For everyone on KDE 4.1, I've created a backport branch here: http://websvn.kde.org/tags/powerdevil/
You can use this, and PowerDevil will work great on your 4.1 installation too!
Well, I'm supposed to talk about anything I want, so, i'll introduce myself briefly: i'm from Italy, 19 years old. I am quite a crazy guy, with a strong passion for music (I sing and play guitar and piano), and for anything that means party and enjoyment!