It works for a different reason than you might think. You may have read that overmixing quick-bread batters causes gluten to develop, resulting in dense, rubbery pancakes and quick breads. However, we’ve found over years of testing that such batters are thin enough that thorough mixing doesn’t actually contribute to toughness.
Rather, creating light, fluffy pancakes and muffins has to do with batter texture and hydration. A batter is more viscous when it’s lumpy than when it’s smooth, since the lumps prevent water from flowing and the mixture from spreading.
Lumpy batter is also better able to hold onto air pockets during cooking, further contributing to height. So, gently stir batter so that lumpy pockets of flour remain, ensuring height in your baked goods. For pancakes, let the batter rest briefly after mixing to allow the unmixed flour pockets to hydrate slightly (a step that is not necessary with, for example, blueberry muffins, which have additional hydration from fruit).