Tetrasodium etidronate is one of the salts of etidronic acid, a chelating agent. Chelating agents can sequester (grab onto) free metal ions or minerals. The analogy we like the most is to describe this material as a lobster, with claws closing around the minerals as they float past in the water.
Removing free metal ions or minerals has several benefits. Firstly, it ensures successful saponification but also stabilises formulas by acting as an antioxidant and therefore prevent changes in colour, texture and fragrance. It also reduces the hardness of water, providing a soft feel to the skin.