So Yami explained what he had learned about himself and Serena. From when they met, meeting her again when she became his shadow. Then to when he had first confessed his love for her and trying to find a way for them to be together without being a secret. Then the battle that led to Zhara's loss of control then to finding out he sealed it to help her till what was inside was dealt with. 

"Are you talking about letting another take control? Most efficient way might be to add that ones power or materials that are bound to that source." Marik explained. "Why?"