Time to put my hand up and say that I'm not sure either of what - or rather which - specific thing is meant by "parallel" in this case, which he has used as a noun, a verb and an adjective. I went through conservatory so am well familiar with music theory, and am able to make logical guesses. So I know what I mean when I use the term. But not, exactly, what Josepp means. Playing in octaves? Using enharmonics to build chords?? My best guess would be playing a unison on two different strings simultaneously (something that harpists do all the time, ditto bowed strings). But that's only speculation.

OK, Josepp, you have all our attentions. Now spill.