Water enters the shower head through the flow resrictor (1) then travels through the injector plate (2) which directs the water to the waterwheel (3). The water spins the magnetic waterwheel past the stator (4) of the field wincing (5). This hydroelectric generator develops the 2.5 volts at .31 amps which lights the PR-6 bulb.
The result? “The Showerstar will be sure to light up your evenings as the perfect addition to any romantic setting.” I doubt it. The kind of person who would buy one of these would probably prefer taking a voltmeter into the shower than a partner.
Link (Thanks, Simon!)