"Brake Energy Regeneration works by harnessing the energy currently lost through engine over-run during braking. The system uses an intelligent alternator, which during braking engages and powers up the battery to 100 per cent with what would have been lost energy. It also reduces drag on the engine by only engaging and charging when required. BMW’s Brake Energy Regeneration system cuts CO2 emissions and fuel consumption by around three percent.

Brake Energy Regeneration is part of the company’s Efficient Dynamics program that aims to reduce vehicle weight, lower fuel consumption and improve efficiency in other areas such as aerodynamics, all with the aim of developing more power from less fuel in a lower-weight vehicle."

http://www.youtube.com/watch?v=nvi0gMyB39M