Complex sentence: All Eco-Drive watches are powered by a solar panel hidden under the face of the watch, which is supplemented by an automatic quartz power source in the rare and discontinued Eco-Drive Duo series.
Split into simpler sentences: All Eco-Drive watches are powered by a solar panel hidden under the face of the watch. The solar power is supplemented by an automatic quartz power source in the rare and discontinued Eco-Drive Duo series.