To perform the glitch:
Be in creative mode.
Put a full stack of an item in the first hotbar slot.
Put any item on the second hotbar slot. (it doesn't need to be a full stack)
Open your inventory, hover over the second item, then press 1. This will swap the two items.
Drop the item in slot 1.

This will drop whatever was in slot 2. If it doesn't, it will join slot 2 when you pick it up.