This isn't an app that you install that constantly runs. It's just a one time utility you run to change settings.
What it does is adjust the registry settings that govern at what percentage of speed the graphics chip runs. By default on laptops, NVIDIA's driver will lower the speed it runs at, which is great for saving battery power at the cost of responsiveness. On the other hand, it still does this even when it's plugged into AC power, which means your graphics chip is potentially running at 100%, 75%, or 50% of speed capacity. CPU's do the same thing, however, unlike the CPU's where it is virtually un-noticeable, I can definately notice when the graphics chip is "shifting gears" on my laptop.
With the default settings, if I grab a window of an open app and move it around, it's slow, and eventually speeds up a little. By contrast, after adjusting the settings with PowerMizer Switch, it consistently runs full speed and fast until you unplug from power, at which point it slows down to save battery power.
You can make a system restore point in XP or Vista/7 and try it out, if you don't like it (which I doubt), you can always restore back.
Another way to avoid this issue is to switch to Windows Basic mode, rather than Windows Aero mode.