[morzh]

Obviously my question has everyone scratching their heads as no one on both forums has been able to tell me anything.

I think the cap between the two contacts, at least in theory, could be useful in the following manner: it lets the sliding contact engage patially charged caps. That is, if the cap is just hanging by itself, its voltage is equal 0, and when connected, it will charge from 0 to full value, whereas when partly charged (you can see how those are lesser values than the bridging cap, thereby charging to the larger portion of the voltage) they will maybe pull less current, create less spark and make less scratchy noise.

I dont have the answer why.
But I will have to stuff it with something, I guess.