I assume from the photo on braingears that the device is attached to the upper radiator hose which is then attached to the thermostat housing. As the water enters the device, it pushes both through the thermostat into the engine and also through the radiator. The old coolant is thereby replaced by the water and is eventually expelled through the radiator drain plug.

Did you order the open thermostat and coupling from the dealer and if so how much did they cost? Thanks.