Why am I not allowed to knead my yeast dough twice?
When kneading the dough, gluten strands are formed. These absorb the moisture from the dough. This is comparable to small bubbles full of water. When the dough is kneaded, the gluten strands are damaged and the moisture is released. The gluten strands ensure that the dough has cohesion and stretches during proofing. Damaged gluten strands make the bread feel 'heavy' and not airy.