Weirdly I did it while reading the post... I think in part because I had had to consciously learn it myself so I was checking the movement (aside: what no-one told me was the sound comes from the impact on the the pad of the thumb, not the two fingers rubbing against each other as I had thought, when I realised that it became easier).

Though [deleted]'s theory makes a lot of sense. In general people reflexively perform actions when prompted. BLINK