I've been told that the punches to the hip and/or rump area, in the clinch and away from the referee are an attempt to aggravate the sciatic nerve. If it works or not I don't know, but at some point I guess it either did, or some one thought it did.
Early in the Toney/Holyfield fight, Toney landed a left hook high up under Holyfield's right arm that made him visibly wince. Max Baer said that Jack Dempsey, in a training camp demonstration, hit his left bicep so hard that his arm was numb for several minutes.