When folding two mixtures together it’s important to always add the lighter mixture (like whisked egg whites) to the heavier mixture (like a chocolate cake base) rather than the other way around. This will not only make the folding action more efficient it will also minimising the air lost when combining. Also, remember that if the heavier mixture is also thick, it’s always a good idea to fold a large spoonful of the lighter mixture through it first to ‘loosen’ to make it easier to incorporate the remaining mixture.