This is the hard part! It takes some very intense stirring to keep the custard smooth instead of just freezing into a solid mass. It especially likes to freeze on the sides and bottom of the pot, so they have to be scraped constantly. The rig seen here allows the pot to be clamped to the table, so that someone doesn't have to hold it (I found that people tended to be put off by the liquid nitrogen that inevitably splashed on them). The nitrogen also has to be poured in a nice consistent stream. Taz has gotten good at that!