There are a million and one recipes on the internet for eggnog, so believe me when I say that I’m under no illusions that what I share here is going to revolutionize the eggnog industry. I know… but you see, I don’t drink eggnog, but my son lurves eggnog! I don’t think he has ever tried it with chocolate flavor and I hope he’ll enjoy them as well as you will.
Answer: Funny expression, isn't it. Either it's an old recipe or one written by someone who isn't a great communicator.
If you've ever eaten Egg Drop Soup, you've seen the perfect example of eggs not being "tempered in" — that is, you've seen eggs intentionally scrambled into a soup.
When you need to smoothly add eggs to liquid ingredients that are already hot, you temper the eggs by adding a small amount of the hot liquid to the bowl containing the eggs while mixing vigorously. By doing this, you raise the temperature of the eggs and dilute them so that their proteins won't coagulate when you add them to the rest of the hot liquid. Once the eggs have been tempered in this way, you can add them to the hot liquid ingredients without having them curdle.
Depending on the volume of eggs and quantity and temperature of the other liquid ingredients, you may need to dilute the eggs more than once, so that you're essentially adding a hot egg soup to a soup, rather than heat-sensitive, eager-to-coagulate eggs to a soup.