We first started doing this back in the late 60's. We got the idea while cranking on the handle of the home made ice cream maker and adding rock salt to the ice in that. It started as a quick way to cool sodas but as the years went by and we got older it was used to cool down our beer. BTW, the more salt you use the faster it cools them off, up to a point. With some trial and error, we refined it to being able to have a light slush in your beer after 7 minutes.