When you decelerate, the energy created is lost. Brake energy recuperation recycles this energy to lighten the load on your engine. It can reduce fuel consumption by up to 3%.
When you’re coasting or braking, the car’s alternator generates energy. Brake energy recuperation allows this energy to be stored. When you accelerate or drive at a constant speed this stored energy is used so less energy is wasted in driving the alternator. This lowers fuel consumption and CO2 emissions.
Article source: www.audi.co.uk