This is incredible. This company has invented saws that can sense the differences in electrical impulses of objects other than wood. There is a high frequency electrical field around the blade and if it's interrupted, the blade stops and retracts in 5 milliseconds. Click on the link and watch the videos, they are crazy. They're no doubt expensive as shit, but if anyone works in construction, im sure these things are the best invention since sliced bread.
http://www.sawstop.com/video.htm
http://www.sawstop.com/video.htm