A counter-pressure bottle filler is designed to deliver a carbonated product into a bottle without excessive foaming. The basic idea behind these devices is to first pressurize the beer bottle to the same pressure as the keg holding the beer. After the bottle is pressurized and the beer valve allowing beer to enter the bottle is opened, the beer will only begin to flow if the keg is placed higher than the bottle.
The beer flow will stop quickly as the forces pushing it from the keg and those slowing it from entering the bottle become equilibrated. When this happens, the bottle vent is opened and the beer flow will continue until the bottle is full. Then, the pressure in the bottle is slowly relieved and the filler is removed from the bottle. If everything went right, there is little foam coming out of the bottle and a cap can be quickly placed on the bottle.