When I was young, I had a clunky, inefficient dynamo attached to my bike that powered the headlight when I pedaled. The Siva Atom works the same way but it converts your pedaling into power to charge USB devices and the removable 1650mAh battery pack.
According to the company, "The resistance is very small, and is proportional to how large your charging load is. At full generating capacity, it is akin to riding with your tires at 70 psi instead of 90 psi, or pedaling up a 0.3% grade. In our use around town, it is very difficult to tell that it's on the bike."