The Potion Shelf is a block added by BiblioCraft. Using a valid potion on the Shelf will place that potion on the shelf in that slot. Right clicking without an accepted potion OR if the shelf is full will open the GUI. Sneak clicking with an empty hand will cause the potion the player is aiming at to pop off. Potions are added/subtracted from whichever slot the player is aiming at.