You've heard the term soft peaks, and you've heard the term stiff peaks. With this recipe, we want soft peaks. What that means is you need to whip the egg whites into soft peaks. Soft peaks will melt back into the mixture before you can even get them out of the mixing bowl, whereas stiff peaks tend to keep their shape. The reason you want soft is because you want them to continue to rise as the cake bakes. If you've over whipped your batter and have stiff peaks, your cake may collapse while baking.