I was about to write something about how they are used and mounted, but you have already guessed thier purpose.
A rather cute idea, until you actually try to use them. I purchased something like them about 20 years ago, and never could figure out why they were better than a simple piece of yarn.
actually, they go on the shrouds well below the spreaders. The curvy part wraps around the shrouds and i put them about 4 feet high above the deck on the the shrouds (also called side stays by some folks).
Yeh, about the same as yarn, but my yarn seems to disappear--birds? Even the yarn I've threaded as luff indicators in the jib luff disappear.
These are easier to spot sometimes than looking in the sun up to the windvane (which also sometimes gets hungup on spiderwebs.
And then sometimes I don't have use of any and its just on what you see, feel and hear.
I seem to see these a lot on the local Flying Scots. I'd guess if you have a foresail that really overlaps the shrouds like a 170% they could get beat up pretty bad.
They were free so its cool, I know I will have to face in towards the mast if I use them, I have a large genoa. to use or not ????? my main has yarn and I also have a set of sail care tape on tell tails so I have plenty of options.