I asked one of my coworkers why they did that test and he did say that they were checking for rebound tenderness which is a sign of peritonitis.
got this info from wikipedia....
Rebound tenderness is a clinical sign that a doctor may detect in physical examination of a patient's abdomen. It refers to pain upon removal of pressure rather than application of pressure to the abdomen. (The latter is referred to simply as abdominal tenderness.)
It represents aggravation of the parietal layer of peritoneum by stretching or moving.
Rebound is regarded as one of the classic local signs of peritonitis which differentiates it from diseases such as appendicitis. The others are tenderness and guarding.
incase you were wondering what peritonitis was...
The more common type of peritonitis, called secondary peritonitis, is caused by the entry of bacteria or enzymes into the peritoneum from the gastrointestinal or biliary tract. Both cases of peritonitis are very serious and can be life-threatening if not treated properly.