Knocking on wood was about knocking on unpainted wood, so that you have actual direct physical contact with wood. Remote knocking seems to go in the other direction...
How do you know this? For example, there is a wikipedia page section entitled "Origin of use in the English language"...
https://en.wikipedia.org/wiki/Knocking_on_wood#Origin_of_use...
but the contents of that section make it extremely clear that no relevant facts are known to wikipedia.
But is is important to mention that a ctrl-f on that page for "unpainted" is proof that you didn't spend much time looking.