Do you know how a starter works? Do you know how a push start works (when someone pushes your car to get your car to run)? They are somewhat relevant to how engine braking saves gas. I don't have a clutch switch on my car...so if I keep my car on first gear, and crank the starter, the car will move forward, since the starter is turning the engine, thus turning the transmission and the wheels.
In a sense, if your car is in MOTION. And your in gear... the motion of your car, the turning of the transmission, will turn the your engine, thus keeping it on. Which uses NO gas.