Centering it allows Yoshiro to regain health.
But what is it?
When Yoshiro is unwell, Soh loses her body completely.
But when she 'dies' or when Yoshiro loses all her health, a body is left behind and this strange substance is all over her mask before she falls.
It looks almost similar to the defilement core.
Since we're helping Yoshiro purify the Torii gate, is the defilement core a form of Soh's?
It might be linked to Yoshiro holding impurity within her.
!! I decided to check what Yoshiro looks like when she's attacked.
It might be the impurity from the Seethe themselves as their attacks cause a build up of impurity, resulting in defilement. You can even see the Seethe's strike is an arc of impurity.