When working with your dough, the last thing you want to do is work it too much. When working with natural yeast you have to work more at protecting what the yeast are creating instead of doing the work yourself.
With many types of bread, pizza, bagels, muffins, etc, you will knead the dough to develop the gluten structure. You can me quite rough with it. But working with sourdough is a different beast. Instead, you want to retain the CO2 the wild yeast are creating in your dough. To ensure this we implement the stretch and fold method.
After mixing your final dough and before you put it away for 12 hours, you have to work a little bit at developing that gluten. To do this take 1/4 of the dough, pull it until you feel resistance. Do not tear the dough. Fold it into the middle of the dough.
Do this four or five times, then invert the dough so the folds you created are now on the bottom. You will do this ever 30 minutes for 90 minutes. After 90 minutes you have developed some gluten while not mashing the dough into smithereens.
The gluten web you form acts like a sail in literally catching the rising CO2 from the yeast and like a hot air balloon and your bread will rise.