The gallbladder contracts and pressurises the biliary system. There is a control valve at the point where the bile duct enters the duodenum and this regulates the flow of bile. The gallbladder when it contracts expands the bile ducts inside and outside the liver. The liver then increases the flow of water and sodium bicarbonate in the liver bile ducts. The pressure in the biliary system peaks and the control valve opens letting out pulses of bile consisting of concentrated bile from the gallbladder and normal liver bile. The flush potion is a clever mix that causes an extreme flow of bile which is responsible for washing out the stones.