Folding is the lightest way to combine two mixtures, particularly when incorporating egg whites. Ingredients such as nuts and vanilla extract are added first to a batter, followed by flour and egg whites. In whisked cakes, melted butter is added last.
To fold the flour into the batter, first tip it into a sieve or sifter. Sift about one-third of the flour over the batter and fold the two together. When almost blended, sift half the remaining flour over the batter, and continue folding.
Sift the remaining flour over the batter and fold it in until the batter is smooth. Scrape the insides of the bowl while folding, as flour tends to cling to it. If folding egg whites into the batter, add them one-third at a time, with the flour.
If adding clarified butter, fold it in as quickly as possible just after the last batch of flour has been added.