The Reelight SL620 LED's flash while your wheels are in motion, crucially they continue to flash for up to 2 minutes after you have stopped thanks to the built-in capacitor which stores current while you are cycling. The SL620 LED light mounts on the seat post of the bike, with the remote sensor on the seat stays to connect with the wheel magnet. The high quality light diodes and the powerful neodynium magnets guarantee quality and durability. With Reelight, you always have lights on your bike day and night! The permanently fitted magnet lights flash when the wheels rotate and work without the need for batteries. When pedalling your bike, friction free Reelight bike lights create their own energy by magnetic induction from the wheel mounted neodymium magnets, powering high efficiency flashing LEDs. The lights are based on the electrodynamic induction principle. Electric current is then produced when the magnets pass the light, which incorporates a spool of copper wire. The light then flashes every time the wheel rotates. Simple, effective and ingenious. The lights are easy to fit all you need is a screwdriver and an adjustable spanner.