I make ice cream. I know the process, how it’s done; you start with a creme anglaise, you chill it, churn air into it to prevent ice crystal formation, etc. This isn’t a foodie question. This is physics.
My ice cream maker is an aluminum cylinder that sits within a plastic tub. Insert ice cream into the metal drum, insert paddles, top with motor. Around the outside edge of the cannister, pour layers of ice and salt. Start motor. Voila, ice cream. So far so good.
On the box of rock salt it says “for best results, draw off excess brine.”
Remembering my grandparents ice cream churn, the ice cream cylinder itself sits low enough in the bucket so that ice can cover the top of it and there is a small hole in the side of the bucket near the top so that water can run out. You want the cylinder to be covered in ice but if the salty water gets too high it can get into the cylinder (I personally like the salty ice cream and everyone else was happy to let me have all of it). Hence the importance of drawing off the excess brine via the drain hole. One of my jobs, when I wasn’t cranking, was to keep that hole clear.
