How SawStop Works
Thanks to Alex for the link:
The SawStop system works by recognizing the difference in the electrical properties of wood and a user. The system induces a high-frequency electrical signal on the blade of a table saw and monitors this signal for changes caused by contact between the blade and a user’s body. The signal remains unchanged when the blade cuts wood because of the relatively small inherent electrical capacitance and conductivity of wood. However, when a user contacts the blade while the saw is operating, the electrical signal changes because of the relatively large inherent capacitance of the user’s body.