Coagulation begins almost instantly when you add cold eggs to a hot liquid, resulting in scrambled eggs in a hot sauce. Heat-sensitive foods such as eggs must be tempered to retain a smooth texture by slowly acclimating them to the higher temperature. Eggs are most frequently tempered with milk or cream for puddings and custards, but other hot additions include batter and even butter for Hollandaise sauce.
Mind Your Temper
Eggs should be tempered in a separate bowl from the pan of hot liquid. It helps to prepare the eggs in the bowl first so they are ready by the time the liquid heats to the proper temperature. Crack open the eggs and whisk them a few times until they are smooth. Fill a ladle or measuring cup with the hot liquid. Drizzle the liquid into the eggs in a slow, steady stream while constantly whisking. Continue adding more liquid a little at a time until the eggs are hot. Stir the hot egg mixture back into the rest of the liquid in the pan. Start at low heat and increase the temperature setting in small increments to slowly heat the mixture. Stir constantly to prevent scorching and keep the mixture smooth. Most egg and milk-based recipes should be removed from the heat just before boiling.
References and ResourcesSerious Eats: How to Temper Eggs
My Recipes: How -- And Why -- Do I Temper Eggs?
Crafty Baking: Temper or Tempering Ingredients
Cooking Light: The Most Common Cooking Mistakes
Baking Bites: How Do You Temper Eggs?