Using the shortcut to swap between items in the hot bar results in 3 ways:
1. using 2 place-able items makes the one you placed down disappear, and the other gets placed down in its place and the item in your inventory gets replaced with a different item of the same type.
2. if you swap one place-able item and something that cannot be placed down(like a tool or diamond) it makes you not able to place it down.
3. if you just use only one place-able item and move it to a different slot in the hot bar, you cannot place it down.
The pictures show that I swap the torch and dust using the "1" key and placing the dust down, so I end up with 2 torches in my inventory and one on the ground.