In the standing position, the blood has to be actively pumped up the column of veins. Fortunately, this is achieved with our calf muscles which squeeze the venous sinuses deeply situated within the leg when we walk. This forces the blood upwards into the pelvis. 

However, when the muscles relax, the blood tends to rush back into the leg. This is called reflux. To prevent all the effort being wasted, there are valves which snap shut holding the column of blood until the next calf muscle contraction occurs. This can be heard with the Hand Held Doppler. When the calf is squeezed, a "whoosh" is heard indicating the blood rushing up the veins into the pelvis and out of the leg. In the normal situation, this is followed by an abrupt silence as the valves close thus stopping the flow.