you'll know it's harder to pedal when the dynamo is engaged than when it's switched off. That's because some of your pedalling energy is being "stolen" by the dynamo and turned into electrical energy in the lights. If you're going along at speed and you suddenly stop pedalling and turn on the dynamo, it'll bring you to a stop more quickly than you would normally, for the same reason: it's stealing your kinetic energy. Now imagine a bicycle with a dynamo that's 100 times bigger and more powerful. In theory, it could bring your bike to a halt relatively quickly by converting your kinetic energy into electricity, which you could store in a battery and use again later. And that's the basic idea behind regenerative brakes!