- Potion Master
- Instant cast
- Allows an alchemist to sometimes create an additional potion when brewing high level potions.
Potion Mastery is a specialization of Alchemy. It allows an alchemist to sometimes create an additional potion when brewing potions.
It appears that the benefit (over time) is about a 15% increase in the number of potions created. Each time it procs, it may result in some number of extra items, typically 1-4 (for 2-5 total). Despite the tooltip, this ability appears to occur when making at least some potions below the Master level.
Obtaining the Potion Master specialization
Prior to Cataclysm
This specialization was directly obtained by completing the quest  from Lauranna Thar'well <Expedition Cataloguer> at the Cenarion Refuge. You needed to be at least level 68 to be offered this quest.
An alchemist who has specialized in one of the other two specializations could switch to this one by talking to the quest giver of their current mastery and pay a 150 fee. The character then had to talk to the quest giver of the mastery desired and instantly get the new mastery. This was possible only after the character has completed the quest to gain one or another of the Mastery abilities.
Patch 4.1.0 (26-Apr-2011): Now obtained by much simpler and easier  quest.
Patch 3.3.0 (08-Dec-2009): [70D] was updated.
Patch 2.1.0 (22-May-2007): Fel Strength Elixirs were corrected to proc via elixir mastery, not potion mastery.