I believe that ADC messages are sent by means of electricity.
I have received ADC's quite frequently over the last 8 years and keep a pretty good diary. One thing I notice is that when they happen during sleep, I usually first get an electrical jolt that startles me out of my sleep.
If I get an ADC when awake, I first feel a current running through by body just before the ADC begins.
I am not a scientist, but it seems to me that ADC's are transmitted by means of electricity, or perhaps radio waves, as I sometimes hear radio type sounds (music and voices). But the music is not contemporary, often sounds like music from the early 1900's.