The Mana Mirror is an alternative to the Mana Tablet, instead of having to keep refilling your Mana tablet when it runs out you can bind a Mana Mirror to a Mana Pool and drain Mana directly. The Mana Mirror can be bound to a Mana Pool via Shift Right/Clicking, the Mana Mirror will keep the Mana Pool chunk loaded.
Be aware that the Mana Mirror will repair items or provide any times you wear with Mana, the Mana pool that is linked needs to be kept full. The Mana Tablet will have a durability bar which will go downtime and eventually display 0 once the Mana pool is empty.
While holding the Mana Mirror you will be able to see a wireframe of the linked Mana pool, this can be seen through walls.
You are also able to have multiple Mana Mirrors that can be linked to different Mana Pools.