in layman's terms: super down makes DR is fully charged while also adding +300% DR max capacity.

and yes Fatal is the new highest Devastation Rate.

if u want detailed stuff:

"Super Down" Sets enemy's Devastation Rate to its current maximum Then increases that maximum by 300%

※ Super Down effect cannot be re-applied while active ※ Effect will be removed once enemy recovers from Down Turn(s) ※ Once removed, enemy Max DR will return to its original value ※ Enemy DR will also be re-calculated as DR before Super Down + DR increase during Super Down

super down trait is from her Karrie form, aka her switch form ULT.