Quickly switching tools copies enchantments for a moment.

If you have a diamond pickaxe with efficiency 5 in one slot and a wooden shovel with no enchantments on it in another slot, when you quickly switch between them and break grass or dirt or etc. you will break it like the shovel has efficiency 5. You can do this with any combination of tools. You can use the efficiency of a tool with another one. I did not test this with other enchantments. But it does not seem to work with unbreaking even if you switch quickly.