Check out the 2021 Fiddlerman “White Christmas” Group Project.
Looks like probably a small laser pointer mounted on the bow or maybe the back of the hand. Not sure what kind of vid or camcorder settings it takes to get that sort of blur you can see on the bow and the player's arm. But that would be what allows the line being drawn by the dot to persist long enough to see the patterns.
Reminds me of a thing I saw once where someone on one of the forums was using a laser pointer taped to the stick back under the adjustment screw to see how straight they were actually tracking the bow when they played.
Back to what is shown in the video, the area the light is shining on would be kinda small for a performance effect. But if you played it onto something like a fresnel lens magnifier that was big enough to catch at least most of the pattern, with the right angle you could have the magnified pattern playing on a wall or screen. At least in a fairly dim venue, it wouldn't be too hard to get the pattern on a wall or screen probably 10 ft across or so. Not great for huge venues maybe, but for small clubs or pubs it could be workable.
Now if you did that with a couple fiddlers/violinists, maybe using a different color for each.. The combined pattern could be interesting enough to maybe catch the audience's attention. It could get some talk going, make a little bit of draw. Could be a fun gimmick for a band with one or more fiddlers/violinists, or for a duet or a soloist with something like guitar or piano accompaniment.
The trail being made by the light wouldn't be as long as a real-life effect as it is on the video, but it could still be a neat background sort of visual effect.
"This young wine may have a lot of tannins now, but in 5 or 10 years it is going to be spectacular, despite the fact that right now it tastes like crude oil. You know this is how it is supposed to taste at this stage of development." ~ Itzhak Perlman