The brakes would still work even if the car was off, the ECU only gives the driver the power brakes. This is a fail safe so that if the engine dies or the power brakes fail for some reason the brakes still work. Power steering works the same way, power when the engine is on, no power when the engine is off. That means if the driver were to push the brakes, they still should apply, although the engine might have enough torque to keep the car moving forward slightly.