Check out the incredible videos of a table saw that can tell the difference between wood and skin. It's something you have to see to believe; I recommend both the regular and super high speed videos.

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.

In the videos, the saw blade is rotating at 4000 rpm. The blade is stopped in less than 5 milliseconds. It can make the difference between needing a Band-Aid or a hand surgeon.