The leg brace creates energy, by kinetic movment, in a magnetic generator. The energy is captured in a battery which then can be plugged into a USB charging device.
As one moves their leg back and fourth energy is exerted by the motion.
Kinetic energy is created by motion. it is energy that is created by the body being in motion. Kinetic energy is able to be captured and converted into another type of energy to power a household object.