A common technique in baking is called “folding“. It is a mixing technique that’s used to combine two ingredients or mixtures with different densities, such as a thick batter and fluffy egg whites. If delicate, airy ingredients aren’t incorporated properly they will lose their loft and baked goods will not rise properly.
When folding, add about one fourth of the lighter ingredients to the top of the batter. Using a rubber spatula, slice down the middle of the mixture to the bottom of the bowl and sweep the bottom, gently lifting and bringing the heavier mixture to the top. The goal is to “fold” the heavy batter up and over the lighter mixture without deflating it.
Rotate the bowl 1/4 turn and repeat. Continue this motion until everything is lightly incorporated. Folding in just one fourth to start allows the rest to be folded in more easily. Add the remaining lighter mixture and continue to fold it in quickly but gently just until everything is evenly incorporated. Over mixing will deflate the lighter mixture.