http://gas2.org/2008/04/22/formula-1...rom-2009-2013/
The biggest difference between KERS and a regular battery-electric hybrid is that KERS stores recovered waste energy in a rotating flywheel. Instead of converting waste energy into electricity and than back into useful energy again with an electric motor, KERS simply transfers the kinetic energy to a ~5kg flywheel in the F1 car’s transmission. The energy stored in the flywheel can then be used by the driver by pushing a “boost” button.