This mod allows for pack makers and players to define items they consider vital in a configuration file that will prevent the player from losing those items on death. That's it, what were you expecting?
The file is in the /PROFILE/config/vitals folder, and is named vital_items.txt
By default, the file is generated with allowing Diamond Armor and Diamond Swords to be kept.
You may use this mod in whatever modpack you please, but please keep in mind that it is in beta and not fully tested for bugs.