Folding in is the technique of combining flour, or another dry ingredient, with a soft, aerated mixture that would be ruined by rough handling — the classic example being a soufflé base or a whisked sponge batter. Because the mixture’s lightness comes from thousands of tiny air bubbles beaten into eggs or cream, the flour must be sifted over the surface and incorporated with slow, deliberate strokes that lift and turn rather than stir. Working in two or three additions helps the flour blend without overmixing, which would deflate the batter and develop toughness from the flour’s proteins. Done properly, the batter stays airy and bakes into a dessert with maximum rise and a fine, even crumb.