3 separate graphite balls in an inert gas atmosphere (Ar+ Kr-85). 1 ball is grounded, and the other 2 connect to the lines being protected.
Under normal conditions, the tube does nothing, but if the voltage on either of the protected lines exceeds a certain value (~150V, IIRC), the gas ionizes, and effectively shorts the line to ground. Once the overvoltage goes away, the gas de-ionizes, and waits for the next surge to come along.
