It's essentially a radiator. Metal does a fantastic job of conducting heat in all directions. A metal frying pan will burn you much faster than a rock at the same temperature, because metal works very fast to equalize its heat. If you've got a 98 degree body radiating heat, and airflow around you, then the metal will do a fantastic job of keeping you cool. Metal is cool, so it absorbed heat from your body, and then the air is cooler so it quickly dissipates heat to the air. The various curves and links of the rings act exactly like a radiator, increasing surface area while also increasing airflow, so it doesn't trap heat, your body still dissipates heat at about the same rate as normal, but it has the added boost of having metal sucking out the heat.
I made a failed belt, and while it wasn't a good belt, I would put my hair in a ponytail and put the belt over my neck when I was outside coiling more wire, and it did a fantastic job of keeping me cool. It gets upwards of 90 degrees here on a normal summer day, there's serious concern for heat stroke to the point where parents have to watch their children and you're discouraged from mowing the grass until sundown. Normally, I'd stand 15 minutes of coiling wire, but with the belt draped over I could go 45 or an hour and do alright.
A metal scarf is really the equivalent of a wool scarf, it's going to do fantastic at regulating heat. It's gotten cooler here now, turned autumn, but come spring I'm going to make sure and have an aluminum scarf ready, probably some sort of sleeves as well. I'd highly suggest it for anyone living in a warm climate, or who enjoys hiking/working out/etc.