The correct fitting of a Mira DCV-H stops water from travelling back up the shower hose and into your water system, should the shower head become submerged in water. This prevents waste water from circulating around your water system, therefore conforming to regulations. A Mira DCV-H (double check valve - high pressure) should be used when a hose retainer/gel hook is not in use or when a fixed handset holder is used.