The Forestry backpacks have four modes. These modes can be toggled by crouching/sneaking and right clicking with the Backpack in hand. The four modes are Normal Mode, Locked Mode, Receiving Mode, and Resupply Mode.
Normal Mode automatically picks up items as long as they are accepted in the Backpack and there is space for them. Shift right-clicking a chest with this mode enabled will dump the Backpack's contents into the chest.
Locked Mode does not pick up items. Chest interaction is the same as in Normal Mode.
Receiving Mode acts similarly to Normal Mode, except that chest interaction will put whitelisted items into the backpack, rather than out.
Resupply Mode acts similarly to Normal Mode in both item pickup and in chest interaction, except it will keep corresponding items in the player's inventory stocked to a full stack as long as possible.
It can be upgraded into the Woven Hunting Backpack.