Puppet Strings is a supplement for FIST that supports more meta style play, leaning into the tropes of thriller fiction with new ways to spend war dice.
The PDF is 10 pages, with a clean layout and some nice historical photographs sprinkled in.
Contents-wise, there's a couple of key mechanics Puppet Strings adds to FIST's formula. The first is BitD style flashbacks, where you spend War Dice to establish details earlier in the plot. However there's also twists of fate (spend a War Die, add a narrative detail, then roll to see if it benefits you,) trivial coincidences (the GM asks a player to establish something true about the world,) dark revelations (major plot elements are connected to a PC specifically, giving them a bunch of War Dice,) and time paradoxes.
Time paradoxes are easily the wildest of the new elements, as they both establish that real world history is immutable (you rewind every time an event breaks canon history,) give the players War Dice, and slowly add random elements as the timeline degrades. They're a great mechanic, but they fully reorient FIST into being a game about time paradoxes.
In addition to these mechanics, there's also some new npcs (including Elvis,) foes, and items---all plenty flavorful.
Overall, this is a great resource for FIST---especially if you want your campaign to be about preserving the timeline. The new mechanics are excellent, and I strongly recommend this as a read for GMs who want to add a little more complexity to their FIST games.