When the transmission signal was seen on the input, the gas would rapidly ionize (helped by a heater coil or radium).Robert Buderi, "The Invention That Changed the World", Touchstone, 1998, p.118. The free electrons in the plasma presented an almost perfect impedance source, blocking the signal from flowing to the output. As soon as the transmission stopped, the gas de-ionized and the impedance disappeared very rapidly. The tiny echoes caused by reflections from the target, arriving microseconds later, were far too small to cause the ionization, and allowed the signal to reach the output.
