The idea behind this system is that you remove the lights from your bike during the day, leaving them where they will get some sunlight (or some semblance of it if you are in the UK...) where they charge up the batteries inside.
Then, when you go to ride your bike, you refit the lights and turn them on. Except they turn themselves on. When the light level drops and/or the lights sense vibration (bike motion) then they come to life. Having both light and movement sensing is a good idea, because bright urban lighting could turn the light off, or standing stationary at the traffic lights could turn it off too. With both, there is little danger of that happening. Considering the technology used, and the size of the lights (not big at all), they're surprisingly bright. On a full day of window sill charging we got just under two hours of pretty good red light action, and just over one hour of white light action at the front.