Castor Oil packs are a way to apply moist heat to an area. The heat acts much the same way acupuncture works, only in a physical and energetic way at the same time. It increases circulation to an area.
Increased circulation will help the body feed cells and tissues the oxygen and nutrients they need and take out wastes/acids. It's like your directing the intelligence of the body to a specific place that needs a little extra attention. :)