Thumper?

[WarEagle57]

So i am trying to decide if I want to use a thumper or not on my pot. My question that i have is about what to fill the thumper with. It will most likely be a mason jar. The website says "the jar is half filled with liquid (water or mash or tails) so that the vapour from the still will bubble up through it; then the vapour coming off it is collected & cooled as per normal. It acts as a second distilling chamber using just the heat from the vapour, and lifts the purity from 50-60% to 70-80%, hence improving what might otherwise be a very mediocre design. Don't make the thumper too small, and start it off with liquid already high in alcohol." So should i fill it with water or mash?

Thanks guys!