I know flashback mechanics from Leverage, but it sounds like they are much the same, given the similar genres.
I think, though, I see some bits here that speak to what I think about when I consider "narrative mechanics". A lot of what we seem to discuss is use of a mechanic in what sounds like otherwise fairly traditional play - the mechanic is being used to gain some added degree of success on a specific test. Largely, then speaking of a mechanic feeding into another mechanic. The narrative control isn't actually the point - the point is to influence a particular tactical situation, most likely a specific success test.
ELIOT: Damnit! There's four guards here. How am I supposed to get past them without a ruckus?
HARDISSON: What, don't you remember the fake FBI badge I handed to you before you left? That'll give you what you need, bro.
But those aren't the only "narrative mechanics" out there. Let's consider some narrative mechanics in which the narrative change is the point. I'll start with one example:
In Fate - let us consider a Conflict (for simplicity, a fight). There are two ways for the conflict to end - the player/GM (not the character/NPC) Concedes, or one of the characters is Taken Out.
If someone is Taken Out, whoever inflicted the final Stress gets to narrate how this happens, and the state of the character removed from the scene - they could be dead, unconscious, fallen off a cliff and their body not found, turned over to the cops, or whatever.
We also note that the narrative choice is not the character's choice. If the villain Takes Out a PC, and the GM may have made it clear that the Villain wants the PC dead, the narration might still be that the PC is knocked out, but alive, because the GM thinks continuing the drama is preferable to just killing the PC.
Similarly, if someone Concedes, the person who did so gets to narrate the state for the character as they depart the scene. It is important to note that this is the player/GM who Concedes - it is not that the PC or NPC in the story is willfully retreating or has given up. So, a reasonable Concession might be that, as they battle the BBEG, rocks fall from the cavern roof, cutting off the villain from the PCs while the villain rages because they still want to try to kill the PCs.
Whatever it is, this narration does not change the success/failure state of the conflict. The conflict was already won or lost. This sets only the narrative/story state of the conflict. It is all about the Narrative.