"An oscillator generates a 12-volt, 200-kilohertz (kHz) pulsed electrical signal, which is applied to a small plate on one side of the blade. The signal is transferred to the blade by capacitive coupling. A plate on the other side of the blade picks up the signal and sends it to a threshold detector. If a human contacts the blade, the signal will fall below the threshold. After signal loss for 25 micro seconds (µs), the detector will fire. A tooth on a 10-inch circular blade rotating at 4000 RPM will stay in contact with the approximate width of a fingertip for 100 µs. The 200-kHz signal will have up to 10 pulses during that time, and should be able to detect contact with just one tooth.[4] When the brake activates, a spring pushes an aluminum block into the blade. The block is normally held away from the blade by a wire, but during braking an electric current instantly melts the wire, similar to a fuse blowing."