Working kind of like a grandfather clock that needs to be wound in order to work, the lamp has a weight that's raised up every day. As the day goes on, it slowly lowers, with that movement powering the LEDs.
I cannot wait to see these things in stores, if they ever make it there. I would love to have a set of lights that never have to be plugged in. With freedom from sockets, the decoration options are unlimited. Bring it on.