HAND is an expansion of the downtime rules from the Ultra book that puts significant mechanical heft into between-missions activity as your merc prep for the next adventure.
It's 50 pages, and most of it is, frankly, a herculean effort in adding specific downtime bonuses and abilities to EVERY SINGLE ONE of the d666 traits. The rules are solid, the trait addenda are evocative, and the helpful Edge Cases section at the end is very useful.
I also like that NinjaDuckie really leans into the bonkers on this - using this supplement RAW, your players can make their characters super beefy (at least at the beginning of the next mission!) to make for even more chaos at the table.
One of the best side-effects of a guide like this is by extending EVERY SINGLE ONE of the d666 traits, it gives players incentives to make different choices and unexpected results if rolling on the table. For the min-maxers it adds complexity, and for chaos gremlins (hello!) it adds an entire new layer of shenanigans.