I think he's referring to a fly that I've seen called "Coch y Bonddu".
Coch y Bonddu is the Welsh name for a particular beetle that I think is the same (or similar to) what we commonly call a "Japanese Beetle" here.
Here is how I first became familiar with it...
Summer of 2002 was when I really got back into fly-fishing; I'd just moved back to the northeast (State College) after a five year stint in Austin, TX and I was hell-bent on making the most of again living in close proximity to good trout streams. (I say "again" because prior to my Texas folly, I spent an absurd portion of my youth scrambling around Enfield Glen in Robert H. Treman State Park - I know there are at least a few of you guys here who are familiar with that stream.)
Until my arrival in SC I'd never fished terrestrials, but that summer I leaned heavily on knowledge gleaned from the guys at FFP; and when the water started to get lower and slower, terrestrials were one of their most common recommendations - so I bought two wet black ants from Steve and hit Spring Creek (with some skepticism, I might add). I remember the day well - I was catching a few on my usual assortment of nymphs, but pickings overall were pretty slim, so I decided to give the wet black ant a try. After hooking fish on my first four casts, I was sold on the fly's effectiveness - "unfortunately" two of the four were pretty big fish, and they snapped me off, leaving my with exactly ZERO wet black ants. So I'm standing in the middle of the stream thinking "Great, now what?"
The next thought was "What do I have in my meager fly supply that might look even a little like a terrestrial?"
I've always been very much a minimalist when it comes to my fly box, and that particular summer, coming off my 5 year trout hiatus, my box was particularly sparse. After about a 2 second scan of my entire available inventory, I noticed the row of ratty looking flies that I'd gotten from my grandfather many years before (none of which I'd ever bothered to fish). Tucked in there was a VERY simple, very tattered looking fly that had what looked like a herl body ribbed with wire, with a sparse brown hackle in front of it - and that was it. I thought "Well, that kinda looks like it could be a beetle, or a fly, or something - let's try it."
I took it out to find that the hook had long since rusted, so I cleaned and sharpened it up as best I could, tied it on, and tossed it up into the spot where the ant had just been so effective. The truth is, I wasn't expecting it to yield any results, but I got a strike almost immediately. It was a memorable experience, because it was the first of many times that I had the pleasure of hooking into what I call Spring Creek's "Freight Trains" - I got a great look at that Brown's broad back as he torpedoed right past my legs on his way downstream. I played him for a few seconds, but I didn't stand a chance - the line soon went limp. When I reeled in, I noticed that the fly was still attached - the old hook had broken off right at the top of the bend.
When I got home that afternoon I scoured as many online fly databases as I could find, until I saw a photo that matched what I'd tied on that day, and the photo identified the fly as the "Coch y Bonddu".
You can see a picture of it here: