A hydraulic Hybrid vehicle is one that harness the breaking power into a pressure chamber called an accumulator. Upon accelerating, the pressure that was stored is then released, aiding the vehicle in the most fuel inefficient stage of operation.
Hydraulic Hybrid
The hydraulic Hybrid system is very simple and straightforward. The animation below shows the basic process of storing pressure to the accumulator, and releasing the pressure to accelerate the vehicle.
The above image was taken from the Next Energy web page which is a very good graphical representation of what a Hydraulic Hybrid vehicle does, and how it works. The overall, big-picture idea is that when the driver is slowing down or breaking, there is a valve that is allowing hydraulic fluid that is stored at low pressure in the "Reservoir" to move to the "Accumulator" at high pressure. The Accumulator allows for kinetic energy to be stored as potential energy through high pressure. When the accelerator pedal is pressed, the pressure is released, and the movement of the fluid from the Accumulator to the Reservoir accelerates the vehicle without applying any force from the engine. When the pressure is dissipated, the engine will rev to keep the vehicle at its constant velocity. More Information