just think about how much energy you waste when you pound your heels into the ground while walking or jogging. you exert a lot of effort fighting gravity to lift your foot and body from the ground, only to let gravity take over, allowing your body to fall forward onto your heel again and wear another layer from the sole of your sneaker.
the kind folks at the mit media lab decided to do something about this. by placing a piezoelectric element or a rotary magnetic generator into the heel of your shoe, you can effectively recycle some of your strut and use it for a mobile device.
it’s like regenerative braking for your feet.