This WA tracks Potion Master procs. Ever wondered just how many big procs you had in a lengthy crafting session? Or whether you really are unlucky with your procs? Want to collect average data over weeks and months? Or find out who the best Potion Master is in your guild? Because data is stored the WA can show you both long term data and current session data (in brackets).
And yes, if your stack is at 19/20 when you get a proc this WA will detect the proc correctly. These procs are easily missed when just casually glancing over the chat because a x2 proc in that situation will be split in half by the game, first completing the stack and then starting a new one.
If you're crafting some potions for a friend and you happened to have some of your own potions lying in your inventory you can use the current session data to find out how many were crafted just now.
If you're potion master on a char with which you raid or pvp but you don't want to see anything displayed while doing so, then click on the WA, navigate to the tab "load", scroll down a bit and tick the box for "Instance Size Type" (twice for multiple options). Finally, choose where you would like to have something displayed (for example "No instance").
FYI - A new session is created when logging on to a char or when reloading. Note that while the WA window is open, no data is collected because all weakauras are paused. The WA only loads on chars with which you are potion master.