The biological change that Kelpiens were taught marked the moment for their ritual culling, which in fact sheds their fear-sensing ganglia and leaves them stronger and unafraid.
Vahar'ai is a process that every Kelpien goes through in adulthood. On Kaminar it was believed to bring madness and death unless the sufferer first gave themselves up to the Ba'ul, and the culling was accepted as part of the Great Balance.
Saru goes through it in An Obol for Charon in 2257, triggered by a dying sphere the Discovery encounters. Sure that he is dying, he asks Michael Burnham to cut off his threat ganglia. They fall away by themselves and he recovers, free for the first time of constant fear.
In The Sound of Thunder he returns home and learns the truth. Kelpiens after vahar'ai were once the predators of Kaminar, and the Ba'ul had kept them from maturing. With the sphere's data, Discovery induces the change in every Kelpien on the planet.