1. Find a Task Area with a loot chest
2. Have full inventory
3. Spam right click the loot chest
4. ???
5. Finish the task in no time as you get progress every time you "open" the chest
(The chest won't open as you have no space for the items)
One example I can give is the Bandit Loot Chest in Brisban Wildlands @ Gnashar Hills:
If you have full inventory you can open it infinity times to get progess at the task until you have space in your inventory (But you can't actually open and loot it)