The wood craftshop at Redstone Arsenal bought a SawStop several months ago...it stops the sawblade in milliseconds after it makes contact with a finger, hand, etc. Here is there explanation:
"The SawStop safety system includes an electronic detection system that detects when a person contacts the blade. The system induces an electrical signal onto the blade and then monitors that signal for changes. The human body has a relatively large inherent electrical capacitance and conductivity which cause the signal to drop when a person contacts the blade. Wood has a relatively small inherent capacitance and conductivity and does not cause the signal to drop."
I recommend you check out their link just to see the demo of how the table works:
http://www.sawstop.com/index.htm