What interesting mechanics exist out there?

I don't mean just "here's a new way to roll combinations of polyhedral dice", or "here's a new theme overlaid on a standard progress tracker", or "here's stress with another name".

I mean, actual new conceptual mechanics that produce new and interesting behaviours in-game. Things like CoC's push rolls, or Slugblaster's Beats/Character Arc, or Blades in the Dark's Flashbacks (these might not be the first games that those appeared in, but the point isn't the game, it's the mechanic).

Interested particularly in what those new mechanics bring to the table in terms of player interactions or story development.