Abdominal rebound tenderness occurs when tissue when you touch the abdominal cavity has pain on the opposite side of the issue. A physician palpates the abdomen to check for rebound tenderness.

Blumberg's sign

Abdominal rebound tenderness occurs when the examiner slowly pushes down on the abdomen, then releases the hand quickly, eliciting a painful reaction (due to the reaction of rebound tenderness, and indicating underlying infection/inflammation in the abdominal cavity).

Guarding indicates the patient's muscular bracing/tension when palpation begins to occur by the examiner.

it may cause more damage

A sign of appendicitis, or some other abdominal affliction. It's an exam a doctor does to gauge abdominal pain/discomfort.