A service rig crew was in the process of unsetting a downhole tubing anchor. The rig manager, floorhand and consultant used two pipe wrenches to rotate the tubing string two to three times. The torque was held by the workers as the driller bounced the string with short, quick jerks in a downward and upward motion. An additional one to two rotations were introduced into the tubing string and the string of pipe was bounced again. The tubing string suddenly completed ¾ of a rotation causing the consultant to lose his grip on the 36” pipe wrench. The rotating pipe wrench struck the floorhand on the back of his hard hat causing him to lose balance and fall on the tubing slips, injuring his abdomen.