Class Active Ability system

I'm sure this has been posted before but I was unable to find discussion focused on this topic. 

Class identities briefing don't explain how class active ability system works. 
I asked my friends if I'm the only one who don't understand how the active ability works or how we open these other perks. They don't know how the system works either. 

Is there some kind of indication on when you can use this ability? 
If there is some indicator for it I would say it must be much clearer on the hud and it should be explained somewhere in tutorials and tooltips. 

I might be just an idiot and missed something entirely but I know I'm not the only one who don't understand how this system works so for sure there is some room for improvement.