HAND is a supplement for FIST that expands the downtime between missions. The vibe is maybe a little bit Midnight Suns.
The PDF is 50 pages (!!) with a clear, well-organized, technical document style layout.
Contents-wise, there's a lot here, but the core mechanics are simple. Between missions you can take actions to gain War Dice, gain temporary weapons or armor, temporarily alter weapons and armor, and permanently boost attributes. So, what else fills the fifty pages of this supplement? A unique downtime action for every Trait, of course.
This is a wildly ambitious thing to add, but it contributes a considerable depth of flavor to HAND. All of the Traits interact with different downtime actions in different ways, and all of these interactions fit the character of the Trait in question. Weaponmaster lets you gain temporary type immunities to weapons. Psychometry lets you automatically locate item hotspots.
Overall, this is a really cool supplement that goes far deeper than I expected. If you want to add a ton of unique content to FIST and give the players a little more flexibility between missions, this is well worth picking up.