That's a clever technique. It looks like it would take a long time and a lot of patience, but simple enough.
The set up he was doing was for a specific area on the lower receiver of a AR. Most applications you'd mask off what you don't want etched throw it in the tank and presto changeo! I came across a video where you use Q-tips to etch a small area like a letter or symbol. We use to do surgical equipment basically the same way, although the equipment was slightly more complex, it was the same principle. Type in the serial number print it out on the stencil cutter, lay it on the piece, hold it taught with your thumb and finger, dip the etching electrode in a solution(smelled like vinegar, but had a fancy label) hold in place 30-40 second and rinsed off the part. I used it one night to anodize my knife, pita doing a 1/4 x 3/4 area at a time, it turned out blotchy, but with a little practice I got it. I always knew I could make something like that at home, but until now never saw the need to pursue it. If anyone is interested I'll post it.