Yinsen failed to persuade Yoruichi, because she said she had learned far too many strange and unusual techniques from him.
"I still haven't figured out the muscle control you mentioned last time—the form of the Reishi that makes up one's body—or the Black Flames of the demon realm. I think researching those is much more interesting than a Zanpakuto!"
Feeling the basic Reishi transform at her command in her hand, becoming whatever form she wished, gave Yoruichi an intoxicating sense that everything was under her control.
By comparison, Zanpakuto had too many uncertainties. Although a blade's abilities were related to a Shinigami's own power and personality, that did not mean everyone liked their "true self."
Yoruichi disliked things with too many uncertain factors!
Yinsen disliked them too, but he did not have that concern. He had absolute control over his "Zanpakuto." How much power his blade could unleash depended on Yinsen's current strength—his Reiatsu. It was completely different from the many Shinigami who comprehended Bankai and relied on their Zanpakuto's evolution to increase their own strength.
In other words, the Tenshintai, a tool specifically used to train Bankai, was not especially valuable to Yinsen.
But he soon discovered that it still had some value...
"Watching Kisuke get beaten so badly by Benihime is pretty entertaining."
Sitting on the edge of the stone platform, Yinsen and Yoruichi leisurely watched Kisuke Urahara below as he was chased and attacked by his own Zanpakuto, Benihime.
The method of using a Tenshintai to comprehend Bankai was simple and brutal: materialize the Zanpakuto's "true body." As long as the Shinigami could defeat the materialized form of their own Zanpakuto, they could force it to grant them the power of Bankai.
But accomplishing that was far from easy. Using a Tenshintai to achieve Bankai proved that