How does vigorous mixing add air to cake batter?

Vigorous mixing incorporates air into cake batter by creating a foam, which acts as a leavening agent to expand the mixture.12

When you beat ingredients like egg whites, the mechanical action folds air into the batter, forming pockets of gas. To achieve a fine, uniform internal structure, these larger pockets of air are rapidly subdivided into smaller, more numerous bubbles using tools such as wire whips or whisks.12

Ingredients like egg whites are particularly effective for this process because they produce strong, voluminous foams that can retain their expanded structure even as the cake dries during the baking process. Once these bubbles are established, they provide the foundation for further expansion; as the cake heats in the oven, water-vapour pressure exerts force on the interior walls of these pre-existing bubbles, causing the batter to rise.12