I know it from Polish usage. In Polish it's "odpukać w niemalowane [drewno]"/"knocking on unpainted [wood]", wood is typically not spoken, the important part is that the wood is unpainted/raw. Superstitious people would especially leave the underside of table unpainted or leave some unpainted wood nearby in living room just for this occasion.

The contents of the article make it clear that "touch wood" is basically synonymous with "knock on wood," making the knocking less important than the touching.

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.

N >= 2