I just watched those two videos - very interesting. Hard to imagine how fast that is, and with only 1 pass. Almost looks like it's a transfer method of transferring blake from stencil to metal, but it says each stencil can be used 1,000 to 3,000 times. That's even more etches than I get from stencils! The video says similar to laser etching. Many of the lower cost laser "etching" doesn't really "etch" away metal for the design, but lays a layer of special paint to top of metal for the design. It looks really good, but isn't anywhere near as permanent as a true etched design that's removed metal to make the design.
For the newbies that haven't etched any, the stencil used for the AC/DC etching is a material with a coating to block the electrolyte (conductive liquid) from contacting the metal. A developer removes the blocking material to make the design, so the electrolyte can only contact metal in pattern of design. Applying a DC current thru the electrolyte to remove a tiny layer of metal, anywhere from .001" to .003" deep is usual. Next step is to change from DC to AC current that will place a layer of black oxide to bottom of etched design to give the nice black look.
The felt (or other material) is to hold the electrolyte while applying the electric current. As cheap as commercial electrolyte is can't imagine the need to homebrew the stuff. I'm sure homebrew works, but suspect commercial might work better? Has anyone compared the two side by side?
