Blumberg sign (or Rebound tenderness positive sign) is elicited by palpating slowly and deeply over a viscus and then suddenly releasing the palpating hand. (2)

 Rebound tenderness is tested for by pressing firmly and steadily on a patient's abdomen for a minute or two, and then releasing your hand suddenly. If he finds this agonizingly painful, the sign is positive. It is an uncomfortable and not a very reliable sign, and is most useful when pressure applied in one place causes rebound pain in another. For example, if pressure in his left lower abdomen causes pain in his right lower abdomen, it suggests appendicitis (Rovsing's sign). Many surgeons use light percussion, which is more accurate and much less cruel than rebound tenderness. (1)


1 - http://www.meb.uni-bonn.de/dtc/primsurg/docbook/html/x2980.html
2 - https://offlineclinic.com/blumbergs-sign-rebound-tenderness/