A party wide buff (not cone) that triggers 33%-40% of the time you cast Blood Flow.
It's a passive ability in the sense that you don't have to click on Balance of Nature.
You do, of course, have to click on Blood Flow for the buff to have a chance to trigger.
At lvl 60 with (probably) 5 feat points: 96stam/2s
At lvl 80 with 5 Feat Points: 123sta/2s
You can cast Blood Flow toward a wall and the Balance of Nature will still stick on the tank if it triggers, if you want to avoid healing aggro but restore the tank's stamina.
Doesn't have to be your highest rank of Blood Flow to trigger Balance of Nature.
Not known if the chance lowers with rank.