Folding is usually done when you want to incorporate a food such as beaten egg whites with another mixture. The reason to fold is to keep from over mixing and losing the fluffiness of egg whites. Always fold the egg whites into the other mixture starting with each in separate bowls. Begin with spooning about 1/3 of egg whites into mixture and gently mixing with a folding motion with a rubber spatula. As soon as it is somewhat incorporated, stop and spoon in another 1/3 amount of egg whites. Finally, fold in the last third of egg whites. Do this very gently and stop as soon as it's incorporated. It is important not to over mix.