When you have a tool, for example axe (see video) equipped in main hand and a shield in off hand. And you switch to the sword, press right click to block with the shield and press the drop button at the same time. the axe drops but the sword dissapears from the inventory. if you hit a mob with the invisible sword, then it still does the same damage as the sword. if you pick the axe up then you dont get 2 axe's in the inventory. just 1