Please Explain In The Eye of the Storm to me

I am attempting to build a Tornado shaman and utilize Maelstrom along with it. There is a node called Eye of the Storm that reads “Whenever you expend a Storm Stack you have a chance to cast Maelstrom.”

What is a “Storm stack” and can someone please give me an example of what “expend” means. How does one “expend” a Storm stack?

Storm stacks are gained with the skill Gathering Storm.
It also has various nodes that can consume those stacks for various effects.

I seriously appreciate this. May I ask… can you gain ALL of the benefits/power of the entire Gathering Storm skill tree without actually using it or having it on your ability bar?

I’m seeing many builds “using” Gathering Storm but they aren’t even on their skill bars?

Not all the tree, but you can gain many benefits from it, depending on the skill.
For example, when I was using storm totem, there is a node in that skill that says it now casts storm bolts from Gathering Storm tree.
There are also similar nodes in other skills.

Mostly, primalists use storm bolts a lot and Gathering Storm is an easy way to scale them, even if you don’t use the skill itself.