William himself wasn't exactly a Soulmancer. The enhancement techniques he knew mainly came from his interactions with some powerful Soulmancers he had met in the past.
Thus, the breathing technique he imparted was actually a combination of his exchanges with these mighty Soulmancers and his own understanding. The focus was on mobilizing the body's life energy and absorbing the scattered divine soul energy from the world.
Life energy can be divided into five types, corresponding to the body's five primary organs, as well as six types of energy related to bodily functions. Typical Soulmancer training starts from the basic life cycle. As their expertise grows, they gradually shift to the vital organs, aiming to increase the body's energy storage and eliminate negative substances within.