Overview:

Stop/Start hybrids are not true hybrids since electricity from the battery is not used to propel the vehicle. However, the Stop/Start feature is an important, energy-saving building block used in hybrid vehicles.

Stop/Start technology conserves energy by shutting off the gasoline engine when the vehicle is at rest, such as at a traffic light, and automatically re-starting it when the driver pushes the gas pedal to go forward.

Pulling Out & Driving:
The gasoline engine does not run when the vehicle is at rest.

When pulling out, the electric starter/generator uses electricity from the battery to instantly start the gasoline engine---the sole source of propulsion for the vehicle.

The generator may also convert energy from the engine into electricity and store it in the battery.

Braking
Stop/Start vehicles use a combination of regenerative and conventional friction braking to slow the vehicle.

In regenerative braking, energy from the wheels turns the electric generator, creating electricity.

Using energy from the wheels to turn the generator slows the vehicle.

The engine shuts off to conserve energy when the vehicle comes to a stop.

Source: www.fueleconomy.gov