Found inside a fluorescent desk lamp being salvaged for possible use as an LED task lamp:
It’s one of the few Underwriter’s Knots I’ve ever seen in the wild. Many recent (i.e., built in the last half-century) lamps pass the cords through a plastic clamp or depend on simple bushings, with some just ignoring the problem.
This anonymous lamp sports the usual Made in China sticker, but also features a genuine-looking UL sticker complete with elaborate holograms, so it may well have been sold by a reputable company. IIRC, it came from a trash can in a Vassar College hallway, back when in-person meetings were a thing; perhaps Vassar required known-good electrical hardware.
The Smell of Molten Projects in the Morning 
I have repaired dozens of older electrical fixtures that had such knots in them and I never knew there was a name for it.
To use such a simple thing instead of a clamp never occurred to me. Thanks.
I do remember seeing a few and tying it once or twice. It does produce a more bulky knot than just a single overhand, which of course is the point of a strain relief. However, there isn’t much room in those sockets as it is. Looking up the history indicates that it has only been around since 1944 and to be used “where rough treatment is expected”. I don’t typically sling my lamps around in a circle over my head, but I do feel like this knot would be suitable for such activities if it ever becomes an Olympic event. Finger crossed!
So what makes this better than the simple overhand knot?
it’s symmetrical, compact and I’d guess easier to undo than an overhand