A more accurate term is loin, not in the sense of "loin cloth" or "fruit of his loins", but rather in the sense of sirloin steak, or tenderloin:
In human anatomy the term "loin" or "loins" refers to the side of the human body below the rib cage to just below the pelvis. It is frequently used to reference the general area below the ribs. -- WikipediaThis area -- which can also be described as the lower torso, "the core" or even "the guts" -- has as its focal point the body's center of mass, known as hara or tanden in Japanese, and dantien in Chinese. With reference to this point Feldenkrais writes in his book, Higher Judo, Ground Work:
All movement of the co-ordinated Master is so performed that one point of his body, lying below his navel and situated vertically above the center of pressure of the standing foot, describes a simple curve at the crucial moment of any throw, or rotates only.Something to work towards.