After using skill A, a new skill B will be available for a short duration (each class will have one?!)
The EX instant consists of 1 substitution (A>B) skill, along with a new passive/buff.
In short, you cast a skill, then a certain EX is changed into EX instant, along with some buffs.
Take note that , some EX Instants share the CD with the skill of origin, while some don’t.
There is no regular pattern as to which skill will turn into EX instant; the skills differ depending on the class.