Ruetoo corrupting fever elementalist: is CF being refreshed and how?

I've watched this Ruetoo build: twitch video. He's using CF on a CI elementalist (with Whispers of Infinity).

It looks like CF is getting refreshed somehow each time he attacks with KB. Oviously he is not spending life but ES, so i thought CF refreshed itself also on ES usage (while the gems states it refreshes on spending life), but I cannot replicare that behaviour.

Am I missing something? What's going on?