Two ways, that are basically the same.
When the cars brake, the braking energy is disspated as waste heat.
The idea is that you connect the brakepads to a small wheel that also touches the brake disk, starting that small wheel to spin. That energy is then transfered to a battery, starting a small electric motor that helps the car accelerate. The idea that when you brake, some of the brake energy is harnessed to charge a battery that will be then discharged at corner exit for extra acceleration. Think kinda like those bike-lights that attach to the front wheel.
The other way, is a mechanical: when the brake pad touches the disc, it starts a small wheel turning (same method as the bike-light), but that is transfered mechanically (via cogs, axels and the like) to a large, heavy disc, and starts THAT spinning. Then when the car starts accelerating, that big spinning disc is connected to the drivetrain, giving extra boost to accelerate.
Think of an upside-down bike: If you crank on the pedal, until the rear wheels is spinning pretty fast, there is a lot of energy stored there, right? if you try to stop it, it can hurt, right? If you right-end-up the bike, it will move forward a bit.
Now think of a rear bike wheel being a lot heavier and/or spinning MUCH faster, and you got the general idea.