Here's what Pioneer's website has to say about it:
"Designed and licensed from the Imaging Science Foundation, Inc. the process allows a trained, certified calibrator to adjust contrast, tint, sharpness and color levels for both day and night time viewing with a high degree of accuracy. Your plasma's standard factory settings are maintained, should you want to switch back to them.
The process takes into account not only the layout and size of the room the plasma is installed in, but also the ambient light levels and direction. You can even have different levels set for day and nighttime viewing and also for each source you have - a cable box or a DVD player, for example.
Once calibrated, the settings are locked to avoid any accidental changes. ISF Day and ISF Night are added to the six preset modes, for a total of eight. So at any time you wish to adjust your settings, all you need to do is press a button on your remote. That's all it takes for you to toggle back and forth between your settings. You now have access to the exact high definition experience you want. And it couldn't be simpler."
Hope that helps.