What is positive offset?

I read Dr. Clark's book but i'm still not sure what the term positive offset means exactly.

From what I heard, it looks like a sound that goes from 1 frequency to none. In the case of the 30 KHz Zapper, it would go from 30 KHz to 0 Hz and then to 30 KHz back in forth.

Is this it?

In other words, even if it's a since wave, all the wave would be visible above the zero line and go in the subzero or negative side like regular alternative current does.

Another thing that I don't understand is the 1,000 Hz vs the 2,500 Hz Zappers.

Accordint to Dr. Clark, no matter what is the frequency, if it has a positive offset (whether it's 10 Hz or 500,000 Hz) it will kill parasites, bacteria and viruses so why some people say that the 2,500 Hz Zapper is better than the 1,000 Hz Zapper?

Thanks