It's what it sounds like, instead of bars or sheets of steel you take one or more pieces of cable, forge-weld them into a bar and then forge it into a blade. Makes a nice 'snakeskin' pattern when it's etched.
It has one real problem in the making that's worse than a standard pattern-welded piece: getting and keeping crap out of the pieces. With standard you can clean the bars, stack them together, heat and flux and the flux both carries out any traces of scale or whatever from between pieces and seals the surface(until you hammer it and drive the stuff out and off). When cable comes to you, brand new it'll be greasy; used, greasy and cruddy with the stuff having worked all through the wires.
Cleaning the stuff out of new isn't too bad, use a good degreaser, soak it in kerosene or whatever. Old cable, it's a bit more involved. I've read of everything from degreaser, boiling it in soapy water and pretty much anything else you can think of. What I used to do was fairly simple, since I used new cable: I burned it off. Be warned, smoky and smelly, but it works. I'd stick about six inches into a low fire and let it burn out, then push the next six in and so forth until the whole piece was done(I usually used about an eighteen inch piece). This does have the bad point of sometimes leaving some ash inside, but the fluxing seemed to take care of it nicely. Also, if you used low heat the grease tended to melt and run out before it burned, leaving the inside pretty clean. Several smiths I've read of would take a day to burn clean and flux a lot of cable, so as to have enough to keep them a while.
To flux it- I used borax- first I'd get the very end to welding heat and set the weld. Then heat a few inches to low red and unwind the section slightly(doesn't take much) and, while still good and hot, sprinkle it heavily with borax; you want the stuff to melt and flow throughout. Repeat heating and fluxing if needed, then twist it back as tight as you can. Move to the next section and repeat.
Remember, the flux does two things: it seals the surface of the steel to keep scale from forming, and it causes impurities that may be trapped between pieces to melt at a lower temperature than otherwise, which means when you hammer it the liquid flux and impurities(if any) spray out. Yes, it will burn the crap out of you, an apron and eye protection highly recommended.
To actually make the billet, I'd heat four to six inches of cable up to welding heat and hammer it from the welded end back the first pass, then turn it 90-degrees and go back up the other way; remember, this has to both set the weld and drive out the flux. When this section cooled I'd inspect, and if well set I'd do the next section.
When I had enough for the purpose, I'd fold it two or more times- usually in a 'Z' shape- then flux and weld that into one piece. Then I'd fold that once more, either in half or three and weld it again. You have to be CERTAIN that you've closed any voids between wires and driven all the flux and any impurities out.
When done, you'll have a solid bar of sufficient mass for the blade you want to make, and forge it out, heat treat and so forth as you would any other.
I haven't made any of this in a while and have no pictures, but I did find this picture at this man's site:
This is a beautiful example, with a section of cable left on to make the hilt.
You can make it as I described, or you can cut pieces of cable and wire or arc-weld them around a bar for a handle(which is the first way I read about this being done). In the latter case, one man I saw some information on took one-inch cable, cut four pieces about four inches long, arc-welded them to a handle and brought the whole mess up to heat and did the billet making with a power hammer(Damn, the times I've wished for one of those...).
Besides a different appearance, some who've used it say they've found blades made this way to be a bit more aggressive in cutting; I never tested for that so I can't say. Although if I'm able to make some more, I'll give it a shot.
That's really about it, fairly simple process. Just remember that bit from Murphy's Laws of Combat: the simple things are never easy.