A fantastic invention. In a normal shower the water is heated by the shower unit itself, producing a weak, tepid stream of water. The power shower relies on your central heating to heat the water. This allows it to devote the energy usually used for water heating into pumping, the result being a wonderful cascade of scalding water that can knock you off your feet.
It's not perfect, since it's very noisy and you need to wait for your water to boil before showering (or it will run out half way through your shower and you will scream as the hot water is replaced instantly by a stream of ice), but We can't go back, we won't go back!