Probably moreso now with smart watches etc. I think the reference was to those strange and alien "young people" who have never worn a watch but use the gesture in any case. I remember with double-decker buses, the two storeys were refereed to as "inside" and "on top", which I always imagined came from the open top buses that come out in the summer in some places. Apparently, though, it's a survivor from the days of trams, but I wonder if it came from stagecoaches before then.
Apologies for the digression ED - this has very little to do with "touch wood". I see pagan references almost everywhere but perhaps not in the way that the parts of buses get named.