I thought of several ways this could be done, and this was the one that I dismissed because "nobody
would sell something that dumb".
In only the friendliest of conditions could this be
performed - you would need to position your spectator's hand "for them" for this to work. Distance,
lighting, you or the spectator moving the slightest bit - this WILL NOT work.
It also
wouldn't look anything like the advertisement shows if you did pull it off.