Execute Critical question

I'm really confused. Perhaps the FO series vets can enlighten me regarding this:

When exactly should the prompt to "execute critical" pop up on the screen? When you're near death, for example? Is it tied to a specific character build feature? While VATS is active, do I need to press yet another key to activate "execute critical"?

I use VATS enough to have my crit meter full, but have yet to have the prompt pop up on any of my firefights.  YouTube videos so far don't show how a player gets the prompt to show, only that it shows up, and they use it.