 Headspace gas pressure (usually carbon dioxide and sometimes nitrogen forced into the "gas in" port) pushes the drink from the bottom of the keg, up the tube, and then out of the "liquid out" port connected to a tap or other dispensing device.The quick-connector fittings which allow hoses to be easily attached and removed from the ports come in two mechanical varieties, namely pin lock and ball lock, which are not interchangeable.Historically, pin lock kegs were used primarily by the Coca-Cola company, while ball lock kegs were used primarily by Pepsi.