The memory is of when Satsuki first donned Junketsu. Releasing him from where he had been held for years, drawing her blood and feeding it to him, Junketsu trying to devour her, and her overcoming that and forcing her will upon him.

She will learn of Junketsu, how to awaken him with her blood, that it's a ravenous monster, that it can be controlled, the term "Kamui," and the name "Kiryuin Satsuki."

Second, what she obtains is Satsuki's indomitable willpower. Saretha is cocky, sure - but Satsuki's willpower is a force of nature and something I would view necessary to her being able to don Junketsu and survive it in the future, as her will overriding his is needed.