Ah yes the beautiful magic of a warm and fragrant loaf!
Now, I don't have the most experience with the Sour Dough in particular, for I have been mostly causing my loaves to rise by using a strong air spell within the bread's center...
However, as far as my understanding goes, the "Starter" is a village of very small germs and/or fairies!
As they construct little caves in the dough, they cause it to become light and fluffy, this makes it extra delicious!
You of course must remember to add ingredients they will like, such as fresh berries or a few colorful petals.
Get back to me once you have procured this, and I shall point you to some very nice recipes I have used myself.
When they call for a "Rising Agent", this is the little guys you have gathered!
You mustn't stir them in (it is quite rude), they will dig their way in themselves once you place them in your bowl.
Best of luck in your culinary endeavor!