This is a maneuver during physical examination to detect Ascites — abnormal fluid within the abdominal (peritoneal) cavity (see Diagram – Gastrointestinal System). With the patient lying on their back, face up, we percuss (tap firmly upon) the area around the belly-button. It will sound hollow, from air in the bowel. Then we have the patient roll onto one side, with the belly-button now downward, and percuss it again. Normally, it should still sound hollow. But if there’s enough ascitic fluid floating around, that will make the percussion note sound dull.
This technique only works once there’s a certain amount of ascites. When there are lesser degrees of fluid, it can only be seen by imaging.