::Ah I now see what you mean. The link is put in incorrect. Frith used an instrument with a locked 3rd bridge in the middle, I saw this before on wiki, but it has been deleted. I can't find it on internet anymore, sorry. It must be in some of his cd-booklet I suppose. The only thing I can't find is that he used homemade instrument, but I can't find an explanation of how it looks like. The Elgart/Yates only mentions the technique, not the words 3rd bridge. page 4 by pinning it between a string and the nut, fretboard, soundboard or saddle of the guitar. If you have a better word than extra bridge, please replace it. I don't know how to call it else.