The fork is to implement features discussed in section titled "Governance changes planned for hardfork 25" in this post: https://hive.blog/hive-139531/@blocktrades/roadmap-for-hive-related-work-by-blocktrades-in-the-next-6-months

In addition to those changes, @howo is also working on some change for the HF (e.g. recurrent payments).

This recent post has an in-depth discussion on how HBD interest works: https://hive.blog/hive-167922/@themarkymark/finding-out-how-does-hbd-interest-work-by-reviewing-code