How did slaked lime improve early soap production?

In early soap production, slaked lime (calcium hydroxide) was used to causticize alkali carbonates found in plant ashes.1

Before this development, soapmakers relied on a mixture of fats and plant ashes, which contained alkali carbonates. This process was relatively slow and inefficient. By introducing slaked lime, soapmakers could convert these alkali carbonates into a more potent caustic lye. This chemical transformation allowed for the easy saponification of neutral fats, significantly improving the efficiency and effectiveness of the soapmaking process.1