Someone help me understand "Invigorated Icon" perk please as I have yet to use it.

So if a banner is down...it says it gives maximum contested health. Now I know that's the white bar which briefly appears as you take damage then starts to deplete. So does that mean you need to have taken damage and then use a stim while the banner is activated? One more thing I saw in a video is someone some how heal a mortal wound. How does that work?