Two things happen in the handle
1. It filters. Water passes through a chamber of mineral filter beads. They are designed to reduce chlorine, impurities and sediment before the water reaches you. Over 2–3 months the beads darken — that is what was going onto your hair. Twist the chamber open and tip in a refill pack.
2. It boosts pressure. The spray plate has many small nozzles. Pushing water through smaller holes increases its speed, so the stream feels stronger while using less water. On UK combi and gravity-fed systems this is the difference you notice first.
What it does not do
It does not soften water. No shower head can remove the calcium and magnesium that make water hard; that needs a whole-house softener. What awa does is filter impurities and change how the water hits you.
Fitting it
Unscrew your current handheld head from the hose. Screw awa One on. Run it for 30 seconds to rinse the beads. Done. Standard ½" thread, the same as every UK shower hose.