UPS released this video detailing how its hydraulic hybrid trucks work.
Essentially, the system uses its pressurized hydraulic fluid to motivate a pump that turns the wheels, propelling the vehicle forward. Under braking, the same pump is used to repressurize the fluid, recapturing some of the lost energy.
Once it drops below a preset level, the gasoline engine kicks in to drive a second pump, which drives the wheels, while repressurizing the system.
What's interesting about this system is that the gasoline engine doesn't appear to have a direct connection to the drive wheels, relying wholly on hydraulics to transmit the power.
Of course, it all makes sense when you see it in action.