Re: the zapper can definitely be felt at much lower frequencies.

Yes, that's what I thought, gearing it to the frequencies that were less feel-able. I recall her saying it might be able to go as low as 5 kHz, but checking back now, I see she said 5 volts not kHz. Sorry! :)

I could see the feeling helping in that folks were more likely to think it was doing something, which it always does, but feel-able might encourage them to use it more, because they can 'feel' it working, maybe?