massless
Inspired
After recovering from my head exploding by the video that @Toopy14 showed. I have some thoughts which are probably not even close to being correct.
Is seems to me that the act of hitting that stringless guitar is somehow altering the existing electromagnetic field and causing interference. This in turn is being amplified (by an incredible degree in this case). I wonder if things like this are at play here:
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
On a side note, I'm not sure this really affects much of the character of the tone because my guess it's being greatly amplified to prove a point. I would think the other aspects already mentioned above are what are really changing the characteristics of the tone.
Arguably the biggest problem with this topic is that very last point 'characteristics of the tone' and how that can be objectively measured without bias.