Siegel explains the device on his blog:
The omnipresence of electromagnetic fields is implied just by simple current flow. We are surrounded by electromagnetic fields which we are producing for information transfer or as a byproduct. Many of those fields are very capacitive and can be harvested with coils and high frequency diodes. Accordingly, I built special harvesting devices that are able to tap into several electromagnetic fields to exploit them. The energy is stored in an usual battery. So you can for example gain redundant energy from the power supply of a coffee machine, a cell phone or an overhead wire by holding the harvester directly into the electromagnetic field whose strength is indicated by a LED on the top of the harvester.