It sounds pretty self-explanatory to me. If someone comes in with fevers, chills, n/v, and they complain of RLQ pain with guarding and rebound tenderness, then they probably have appendicitis. They pain would be in proportion to the exam. If someone comes in without all those symptoms and just complains of guarding and rebound tenderness (I don't know if ischemic colitis presents this way, but just for purposes of explanation) then that would be "pain out of proportion to exam." Basically, if they have abdominal pain, but no other causes can be found on PE, IC should be on your DDx.