We want it to be so hot that the doughnuts instantly sear. This prevents them from absorbing too much oil & being soggy.
By the time you have put them all in, some may already be ready to turn. It takes less than a minute for them to be ready to turn over.
Instead of cinnamon sugar, you can also use granulated sugar or sprinkles. Also you can glaze them or spread chocolate on them (without the margarine)!