to communicate with the ECU the way Ford did it in the first place. You can modify the input signals resistances and a couple other means of changing the input the computers read; BUT YOU REALLY HAVE TO KNOW WHAT YOU'RE DOING!
SCT approved shops are really good (typically) at getting a good safe tune out of your car, making them the best way to do it. If you bolt too many items on, you'll notice that is doesn't feel like much is happening... a good dyno tune: works WONDERS!
Coal Country Cruise... YOU MISSED IT!!! Pics here: http://www.focusfanatics.com/forum/showthread.php?t=296590&page=2