I've seen a singer conduct the audience when slowing down at the end of a final chorus, which seems fair enough, and I've also seen one making hand movements (palm up/palm down etc.) during a chorus, which are intended to indicate which way the tune is going. I believe the latter is a common technique when conducting choirs in rehearsal.

But conducting yourself when others are not expected to be joining in? I don't get that, though I'm sure Jack's Indian/Gypsy/Transylvanian example is perfectly genuine.