Folding egg whites into batter helps to incorporate air into the batter, making the resulting cake lighter and fluffier. To do this, add about half of your egg whites to the batter. Starting at the bottom of the bowl, scrape the spatula up the side of the bowl, and then "fold" the batter over the egg whites. Repeat this motion until the egg whites are incorporated into the batter, being careful not to overmix. Once the first portion of egg whites has been fully incorporated, add the remaining egg whites.