Drain hole running clear. Melt goes out the side, not up the can.
Plain ice sits at 32°F and will not budge lower — it just melts and stays 32. Sprinkle rock salt on it and the salt dissolves into the meltwater, and that brine can happily be liquid down around 10°F. It's colder than the ice was, and it wets the whole outside of the can instead of touching it in a few spots. That's the trick. Two-thirds of the job is done before the crank turns once.
It is not a spoon. The custard freezes only where it touches the cold tin, so a thin film sets on the inside wall of the can and the dasher's wood scraper blade shaves it off and folds it into the warm middle. Stirring the middle does nothing. Scraping the wall does everything.
Whip the handle and the custard just sloshes — the blade skids past without a bite, the film keeps sitting on the tin, and film that sits grows. Big crystals. Sandy. Steady middling speed, about forty to sixty turns of the crank a minute, gets the blade into the film every pass and folds air in besides.
Nothing in the recipe decides smooth or gritty. The crystal size decides it, and crystal size is only ever a record of how long the film sat there before somebody scraped it. Under about forty microns your tongue can't find the edges. Past eighty it's sand.
When the handle starts fighting you, that is the batch telling you it's frozen — the resistance is the reading. Stopping right when it gets hard is stopping right before it's ready. Ten more. Then ten more.
Melt has to run out. If the drain packs with ice chips, the brine climbs the bucket and gets over the can's lid, and then you've made salt custard and you can throw the whole six quarts in the alley. Poke it clear with the hammer handle every so often.
Pull the dasher, press the custard down, cork the hole in the lid, bury the can in salt ice under newspaper and sit on the step an hour. It goes from soft-set to firm and the crystals stop growing because there's nothing left liquid to grow into.