This thread shed some light on "flashover," but still doesn't have a lot of detail.
http://dynacotubeaudio.forumotion.com/t2525-rectifier-tube-flash"The diode mod prevents reverse current flashover like when the amp is suddenly turned off, caps are fully charged up, and the rectifier cathode gets cooler losing its negative charge. The conduction path in this special case (no diodes) is back through the heater windings to ground which is very low impedance."
I don't think I understand. How does the current get through the heater windings to ground?